A. Schema format

HED schema developers generally do initial development of the schema using .mediawiki format. The tools to convert schema between .mediawiki and .xml format are located in the hed.schema module of the hedtools project of the hed-python repository located at
https://github.com/hed-standard/hed-python. All conversions are performed by converting the schema to a HedSchema object. Then modules wiki2xml.py and xml2wiki.py provide top-level functions to perform these conversions. This appendix presents the rules for HED base and library schema in .mediawiki and .xml formats.

A.1. Mediawiki file format

The rules for creating a valid .mediawiki specification of a HED schema are given below. The format is line-oriented, meaning that all information about an individual entity should be on a single line. Empty lines and lines containing only blanks are ignored.

A.1.1. Overall file layout

Overall layout of a HED MEDIAWIKI schema file.

             . . .
!# start schema
!# end schema
!# end hed

A.1.2. The header-line

The first line of the .mediawiki file should be a header-line that starts with the keyword HED followed by a blank-separated list of name-value pairs.

Allowed HED schema header parameters






Name of library used in XML file names.

The value should only have alphabetic characters.



A valid semantic version number of the schema.






xsi:noNamespaceSchemaLocation points to XSD file.

The xsi attribute is required if xmlns:xsi is given. The library and version values are used to form the official xml file name and appear as attributes in the <HED> root of the .xml file. The versions of the schema that use XSD validation to verify the format (versions 8.0.0 and above) have xmlns:xsi and xsi:noNamespaceSchemaLocation attributes.

Example: Version 8.0.0 of the HED MEDIAWIKI schema.

HED version="8.0.0"

The version line must be the first line of the .mediawiki file. The schema .mediawiki file is HED-schema-8.0.0.mediawiki found in https://github.com/hed-standard/hed-specification/tree/master/hedwiki.

Example: Version 8.0.0 of the HED XML base schema.

<HED version="8.0.0">

The file name in hedxml in hed-specification is HED8.0.0.xml.

Example: Version 1.0.2 of HED test library in MEDIAWIKI format.

HED library="test" version="1.0.2"

The resulting XML root is:

Example: Version 1.0.2 of HED test library schema in XML format.

<HED library="test" version="1.0.2">

The file name in hedxml in the HED schema library test is HED_test_1.0.2.xml.

Unknown header-line attributes are translated as attributes of the HED root node of the .xml version, but a warning is used when the .mediawiki file is validated.

A.1.3. Schema section

The beginning of the HED specification is marked by the start-line:

!# start schema

An arbitrary number of lines of informational text can be placed between the header-line and the start-line. Older versions of HED have a CHANGE_LOG as well as information about the syntax and rules. New versions of HED generate a separate change log file for released versions.

The end of the main HED-specification is marked by the end-line:

!# end schema

The section separator lines (!# start schema, !# end schema, !# end hed) must only appear once in the file and must appear in that order within the file. A section separator is a line starting with !#.

The body of the HED specification consists of two types of lines: top-level node-specification specifications and other node specifications. Each specification is a single line in the .mediawiki file. Empty lines or lines containing only blanks are ignored. The basic format for a node-specification is:

node-name  <nowiki>{attributes}[description]</nowiki>

Top-level node names are enclosed in triple single quotes (e.g., '''Event'''), while other node names have at least one preceding asterisk (*) followed by a blank and then the name. The number of asterisks indicates the level of the node in the subtree. HED-3G node names can only contain alphanumeric characters, hyphens, and under-bars. They cannot contain blanks and must be unique. HED (2G) and earlier versions allow blanks. Everything after the node name must be contained within <nowiki></nowiki> tags. Placeholder nodes have an empty node name, but are followed by a # enclosed in <nowiki></nowiki> tags.

Example: Different types of HED node specifications.


'''Property''' <nowiki>{extensionAllowed} [Subtree of properties.]</nowiki>


***** Duration <nowiki>{requireChild} [Time extent of something.]</nowiki>


****** <nowiki># {takesValue, unitClass=time,valueClass=numericClass}</nowiki>

The Duration tag of this example is at the fifth level below the root of its subtree. The tag: Property/Data-property/Data-value/Spatiotemporal-value/Temporal-value/Duration is the long form. The placeholder in the example is the node directly below Duration in the hierarchy.

A.1.4. Other sections

After the line marking the end of the schema (!# end schema), the .mediawiki file contains the unit class specifications, unit modifier specifications, value class specification, the schema attribute specifications, and property specifications. All of these sections are required starting with HED version 8.0.0 and must be given in this order.

Unit classes specify the kind of units are allowed to be used with a value that is provided for a # value. The unit class specification section starts with '''Unit classes''' and lists the type of unit at the first level and the specific units at the second level.

Example: Part of the HED unit class specification for time.

'''Unit classes''' 
* time <nowiki>{defaultUnits=s}</nowiki> 
** second <nowiki>{SIUnit}</nowiki> 
** s <nowiki>{SIUnit, unitSymbol}</nowiki> 

The unit classes can be modified by SI (International System Units) sub-multiples and super-multiples. All unit modifiers are at level 1 of the .mediawiki file. Unit modifiers have either the SIUnitModifer or the SIUnitSymbolModifer to indicate whether they are regular modifiers or symbol modifiers.

Example: Part of the HED unit modifier specification.

'''Unit modifiers''' 
* deca <nowiki>{SIUnitModifier} [SI unit multiple for 10^1]</nowiki> 
* da <nowiki>{SIUnitSymbolModifier} [SI unit multiple for 10^1]</nowiki>

Units that have the SIUnit attribute can be modified by any unit modifier that has the SIUnitModifier. So for example, second and decasecond are valid time units as are seconds and decaseconds. Similarly, units that have the SIUnit and unitSymbol modifiers can be modified with unit modifiers that have the SIUnitSymbolModifier attribute.

Value attributes give rules about what kind of value is allowed to be substituted for # placeholder tags.

Example: Part of the HED value class specification.

'''Value classes'''
* posixPath <nowiki>{allowedCharacter=/,allowedCharacter=:}[Posix path specification.]</nowiki> 

The schema attributes specify other characteristics about how particular tags may be used in annotation. These attributes allow validators and other tools to process tag strings based on the HED schema specification, thus avoiding hard-coding particular behavior.

Example: Part of the HED schema attribute specification.

'''Schema attributes'''
* allowedCharacter <nowiki>{valueClassProperty}[Attribute of value classes specifying a special character that is allowed in expressing the value of a placeholder.]</nowiki>
* defaultUnits <nowiki>{unitClassProperty}[Attribute of unit classes specifying the default units for a tag.]</nowiki> 

Notice that in the above example, the schema attributes, themselves have attributes referred to as HED schema properties. These schema properties are listed in the Properties section of the schema.

Example: Part of the HED schema property specification.

* valueClassProperty <nowiki>[Attribute is meant to be applied to value classes.]</nowiki> 

A.2. XML file format

The XML schema file format has a header, prologue, main schema, definitions, and epilogue sections. The general layout is as follows:

XML layout of the HED schema.

<?xml version="1.0" ?>
<HED library="test" version="0.0.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://github.com/hed-standard/hed-specification/raw/master/hedxml/HED8.0.0-beta.3.xsd">
<prologue>unique optional text blob</prologue>
         ...  schema specification  ... 
   <unitClassDefinition> ... </unitClassDefinition>
   <unitClassDefinition> ... </unitClassDefinition>
   <unitModifierDefinition> ... </unitModifierDefinition>
   <unitModifierDefinition> ... </unitModifierDefinition>
    <valueClassDefinition> ... </valueClassDefinition>
    <valueClassDefinition> ... </valueClassDefinition>

   <schemaAttributeDefinition> ... </schemaAttributeDefinition>
   <schemaAttributeDefinition> ... </schemaAttributeDefinition>

    <propertyDefinition> ... </propertyDefinition>
    <propertyDefinition> ... </propertyDefinition>

<epilogue>unique optional text blob</epilogue>

The <prologue>xxx</prologue> and <epilogue>xxx</epilogue> elements are meant to be treated as opaque as far as schema processing goes. In earlier versions of HED the prologue section contained a Change Log for the schema as well as some basic documentation of syntax. The epilogue section contained additional metadata to be ignored during processing. The following subsections give a more detailed description of the format of these sections.

A.2.1. The schema section

The schema section of the HED XML document consists of an arbitrary number of <node></node> elements enclosed in a single <schema></schema> element.

Top-level XML layout of the HED schema.

    <node> ... </node>
    <node> ... </node>

A <node> element contains a required <name> child element, an optional <description> child element, and an optional number of additional <attribute> child elements:

XML layout HED node element.

    <attribute> ... </attribute>
    <attribute> ... </attribute>
    <attribute> ... </attribute>   
    <node> ... <node>

The <name> element text must conform to the rules for naming HED schema nodes. It corresponds to the node-name in the mediawiki specification and must not be empty. A # value is used to represent value place-holder elements.

The <description> element has the text contained in the square brackets [] in the .mediawiki node specification. If the .mediawiki specification is missing or has an empty [], the <description> element is omitted.

The optional <attribute> elements are derived from the attribute list contained in curly braces {} of the .mediawiki specification. An <attribute> element has a single non-empty <name></name> child element whose text value corresponds to the node-name of attribute in the corresponding .mediawiki file. If the attribute does not have the boolProperty, then the <attribute> element should also have one or more child <value></value> elements giving the value(s) of the attribute. Example: The requireChild attribute represents a boolean value. In the .mediawiki representation this attribute appears as {requireChild} if present and is omitted if absent.

The requireChild attribute represents a boolean value.

Old xml if true:

<node requireChild="true"><name>xxx</name></node>

New xml if true:


The suggestedTag attribute has a valid HED tag value. In the mediawiki representation this attribute is omitted if absent and appears when present as shown in this example.

The suggestedTag attribute has a valid HED tag value.


The suggestedTag attribute is meant to be used by tagging tools to suggest additional tags that a user might want to include. Notice that the suggestedTag values are valid HED tags in any form (short, long, or intermediate).

The suggestedTag old format.

Old xml if present:

<node suggestedTag="Sweet,Gustatory-attribute/Salty">

New xml if present:


A.2.2. Unit classes

The valid HED-3G unit classes are defined in the <unitClassDefinitions> section of the XML schema file, and valid HED-3G unit modifiers are defined in the <unitModifierDefinitions> section. These sections follow a format similar to the <node> element in the <schema> section:

XML layout of the unit class definitions.

    <unitClassDefinition> ... </unitClassDefinition>
    <unitClassDefinition> ... </unitClassDefinition>

The <unitClassDefinition> elements have a required <name>, an optional <description> and an arbitrary number of additional <attribute> child elements. These <attribute> elements describe properties of the unit class rather than of individual unit types. In addition, <unitClassDefinition> elements may have an arbitrary number of <unit> child elements.

XML layout of the unit class definitions.

    <description>Temporal values except date and time of day.</description>
       <description>SI unit second.</description>
       <description>SI unit second in abbreviated form.</description>

A.2.3. Value classes

Value classes are defined in the <valueClassDefinitions> section of the XML schema file. These sections follow a format similar to the <node> element in the <schema>:

XML layout of the unit class definitions.

    <valueClassDefinition> ... </valueClassDefinition>
    <valueClassDefinition> ... </valueClassDefinition>

A.2.4. Schema attributes

The <schemaAttributeDefinitions> section specifies the allowed attributes of the other elements including the <node>, <unitClassDefinition>, <unitModifierDefinition>, and <valueClassDefinition> elements. The specifications of individual attributes are given in <schemaAttributeDefinition> elements.

XML layout of the schema attribute definitions.

    <schemaAttributeDefinition> ...</schemaAttributeDefinition>
    <schemaAttributeDefinition> ... </schemaAttributeDefinition>

The individual <schemaAttributeDefinition> elements have the following format:

XML layout of the schema attribute definitions.

    <description>An attribute of value classes indicating a special character that is allowed in expressing the value of that placeholder.</description>

A.3. Schema sections

This section gives information about how the various auxiliary sections of the HED schema are used to specify the behavior of the schema elements.

A.3.1. Schema properties

The property elements indicate where various schema attributes apply. Their meanings are hard-coded into the schema processors. The following is a list of schema attribute properties.

Summary of unit classes and units in HED 8.0.0 (* indicates unit symbol).




Indicates schema attribute values are either true or false.


Indicates schema attribute only applies to unit classes.


Indicates schema attribute only applies to unit modifiers.


Indicates the schema attribute only applies to value classes.


Alphanumeric characters, blank,
+, -, :, ;, ., /, (, ), ?, *, %, $, @, ^, _

Notes on rules for allowed characters in the HED schema.

  1. Schema attributes with the boolProperty have a <name> node but no <value> node in the XML. Presence indicates true.

  2. Schema attributes with the boolProperty have both <name> and <value> nodes in the XML.

A given schema attribute can only apply to one type of element (node, unitClassDefinition, unitModifierDefinition or unit). Attributes that don’t have one of unitClassProperty, unitClassProperty or unitProperty are assumed to apply to node elements.

A.3.2. Schema attributes

As mentioned in the previous section schema attributes can only apply to one type of element in the schema as indicated by their property values. Tools hardcode processing based on the schema attribute name. Only the schema attributes listed in the following table can be handled by current HED tools.

Schema attributes (* indicates attribute has a value).






Specifies a character used in values of this class.



Specifies units to use if placeholder value has no units.



A tag can have unlimited levels of child nodes added.



Event-level HED strings should include this tag.



A HED tag closely related to this HED tag.



A child of this node must be included in the HED tag.



Event-level HED string must include this tag.



This unit represents an SI unit and can be modified.



Modifier applies to base units.



Modifier applies to unit symbols.



Tag could be included with this HED tag.



Tag can only appear inside a tag group.


node #

Placeholder (#)should be replaced by a value.



Tag (or its descendants) can be in a top-level tag group.



Tag or its descendants can only occur once in
an event-level HED string.


node #

Unit class this replacement value belongs to.



Unit is a prefix (e.g., $ in the currency units).



Tag is an abbreviation representing a unit.


node #

Type of value this is.

Normally the allowed characters are listed individually as values of the allowedCharacter attribute. However, the word letters designates upper and lower case alphabetic characters are allowed. Further, the word blank indicates a space is an allowed character, and the word digits indicates the digits 0-9 may be used in the value.

If placeholder (#) has a unitClass, but the replacement value for the placeholder does not have units, tools use the value of defaultUnits if the unit class has them. For example, the timeUnits has the attribute defaultUnits=s in HED 8.0.0. The tag Duration/3 is assumed to be equivalent to Duration/3 s because Duration has defaultUnits of s.

The extensionAllowed tag indicates that descendents of this node may be extended by annotators. However, any tag that has a placeholder (#) child cannot be extended, regardless of extensionAllowed since it single child is always interpreted as a user-supplied value.

Tags with the ‘required’ or ‘unique’ attributes cannot appear in definitions.

In addition to the attributes listed above, some schema attributes have been deprecated and are no longer supported in HED, although they are still present in earlier versions of the schema. The following table lists these.

Schema attributes (* indicates attribute has a value).

Schema attribute




node #

Indicates a default value used if no value is provided.



Indicates where this tag should appear during display.



Indicates the relationship of the node to its parent.

The default attribute was not implemented in existing tools. The attribute is not used in HED-3G. Only the defaultUnits for the unit class will be implemented going forward.

The position attribute was used to assist annotation tools, which sought to display required and recommend tags before others. The position attribute value is an integer and the order can start at 0 or 1. Required or recommended tags without this attribute or with negative position will be shown after the others in canonical ordering. The tagging strategy of HED-3G using decomposition and definitions. The position attribute is not used for HED-3G.

The predicateType attribute was introduced in HED-2G to facilitate mapping to OWL or RDF. It was needed because the HED-2G schema had a mixture of children that were properties and subclasses. The possible values of predicateType were propertyOf, subclassOf, or passThrough to indicate which role each child node had with respect to its parent. The schema vocabulary redesign of HED-3G eliminated this issue. The attribute is ignored by tools.

A.3.3. Value classes

HED has very strict rules about what characters are allowed in various elements of the HED schema, HED tags and the substitutions made for # placeholders. These rules are encoded in the schema using value classes. When a node name or placeholder substitution is given a particular value class, that name or substituted value can only contain the characters allowed for that value class.

The allowed characters for a value class are specified in the definition of each value class. The HED validator and other HED tools may hardcode information about behavior of certain value classes (for example the numericClass value class). HED does not allow commas or single quotes in any of its values.

Rules for value classes.

Value class

Allowed characters


digits, T, :, -


alphabetic characters, -


digits, ., -, +, E, e


As yet unspecified


Alphanumeric characters, blank,
+, -, :, ;, ., /, (, ), ?, *, %, $, @, ^, _

Notes on rules for allowed characters in the HED schema.

  1. Commas are not allowed in any values.

  2. Date-times should conform to ISO8601 date-time format YYYY-MM-DDThh:mm:ss.

  3. Any variation on the full form of ISO8601 date-time is allowed.

  4. The name class is for schema nodes and labels.

  5. Values that have a value class of numericClass must be valid fixed point of floating point values.

  6. Scientific notation is supported with the numericClass.

  7. The text class is for descriptions, mainly for use with the Description/ tag.

  8. The posix path class is yet unspecified and currently allows any characters besides commas.

A.3.4. HED unit classes

Unit classes allow annotators to express the units of values in a consistent way. The plurals of the various units are not explicitly listed, but are allowed as HED tools uses standard pluralize functions to expand the list of allowed units. However, Unit symbols represent abbreviated versions of units and cannot be pluralized.

Nodes with the SIUnit modifier may be prefixed with multiple or sub-multiple modifiers. If the SI unit does not also have the unitSymbol attribute, then multiples and sub-multiples with the SIUnitModifier attribute are used for the expansion. On the other hand, units with both SIUnit and SIUnitModifier attributes are expanded using multiples and sub-multiples having the SIUnitSymbolModifier attribute.

Note that some units such as byte are designated as SI units, although they are not part of the standard.

Unit classes and units in HED 8.0.0 (* indicates unit symbol).

Unit class

Default units







radian, rad*, degree



metre^2, m^2*



dollar, $, point



hertz, Hz*



dB, candela, cd*






byte, B



metre, m*, inch, foot, mile



m-per-s*, mph, kph



second, s*, day, minute, hour



metre^3, m^3*



gram, g*, pound, lb

A.3.5. HED unit modifiers

The unit modifiers are can be applied to SI base units to indicate multiples or sub-multiples of the unit. Unit symbols are modified by unit symbol modifiers, whereas non symbol SI units are modified by unit modifiers.

Unit modifiers (* indicates unit symbol modifier).

Schema attribute


deca, da*

SI unit multiple representing 10^1

hecto, h*

SI unit multiple representing 10^2

kilo, k*

SI unit multiple representing 10^3

mega, M*

SI unit multiple representing 10^6

giga, G*

SI unit multiple representing 10^9

tera, T*

SI unit multiple representing 10^12

peta, P*

SI unit multiple representing 10^15

exa, E*

SI unit multiple representing 10^18

zetta, Z*

SI unit multiple representing 10^21

yotta, Y*

SI unit multiple representing 10^24

deci, d*

SI unit submultiple representing 10^−1

centi, c*

SI unit submultiple representing 10^−2

milli, m*

SI unit submultiple representing 10^−3

micro, u*

SI unit submultiple representing 10^−6

nano, n*

SI unit submultiple representing 10^−9

pico, p*

SI unit submultiple representing 10^−12

femto, f*

SI unit submultiple representing 10^−15

atto, a*

SI unit submultiple representing 10^−18

zepto, z*

SI unit submultiple representing 10^−21

yocto, y*

SI unit submultiple representing 10^−24