Labels and documentation

A period is a skos:Concept, “an idea or notion,” as the SKOS Reference puts it. We use a number of SKOS properties to describe periods:

• skos:prefLabel is used for the name of the period exactly as given in the original source. The value of this property is a simple literal xsd:string, with no language tag.

• skos:altLabel is used for language-specific names of the period, assigned by PeriodO curators. The value of this property is a language-tagged string. The language tag consists of (at least)

  1. a three-character primary language subtag, as defined in ISO 639-2 or ISO 639-3, and

  2. a four-character script subtag, as defined in ISO 15924.

  There will always be at least one skos:altLabel, with the language tag eng-latn. If the source of the period was not written in English, there will always be another skos:altLabel with a language tag indicating the language and script of the source of the period. The language of the source of the period is indicated via a dc:language property, the value of which is a language tag as described above.

• skos:note is used for notes about the period taken from or attributed to the original source. For example, the original Pleiades definition of “Ottoman Rise (AD 1300-1453)” includes the note “ends with the conquest of Constantinople.” The value of this property is a simple literal xsd:string, with no language tag.

• skos:editorialNote is used for administrative or editorial notes added by the PeriodO curators; these do not appear in the original source. The value of this property is a simple literal xsd:string, with no language tag.

• skos:inScheme is used to link a period to the authority of which it is a part.

Source

Usually the bibliographic information about the source of a period is provided through properties of the authority to which it belongs. However, in some cases there may be additional bibliographic information that is specific to an individual period. In these cases, we use a dcterms:source to provide this additional information, and dcterms:isPartOf to link the source of the period to the source of the authority to which it belongs. For example, we might use the following to indicate that the specific book page from which a period was sourced:

<p0tns5v4kdf>
    dcterms:source [
        dcterms:isPartOf <http://www.worldcat.org/oclc/63807908> ;
        bibo:locator "page 25"
    ] .

(bibo is the Bibliographic Ontology.)

Temporal extent

We use properties from the Time Ontology to describe the temporal extent of periods. A period is a time:ProperInterval, an interval of time with different beginning and end points. We assume that these (instantaneous) beginning and end points can never be precisely identified, hence our descriptions focus on describing the intervals that start and finish the period:

• time:intervalStartedBy links the period to an (anonymous) time interval that has the same (unknown) beginning point as the period, and an (unknown) end point that comes before the end point of the period. We call this the start interval for the period.

• time:intervalFinishedBy links the period to an (anonymous) time interval that has the same (unknown) end point as the period, and an (unknown) beginning point that comes after the beginning point of the period. We call this the stop interval for the period.

We describe the start and stop intervals in two ways. Both ways of describing the interval are required; these are complementary descriptions, not alternatives:

1. skos:prefLabel is used to textually describe the interval exactly as given in the original source, for example “end of the first century BC”. The value of this property is a simple literal xsd:string, with no language tag.

2. time:hasDateTimeDescription is used to describe the interval in a more structured fashion. This property links the interval to an (anonymous) time:DateTimeDescription. These structured descriptions are created by PeriodO curators.

Currently we use the following properties in our datetime descriptions:

• time:year for descriptions of intervals that can be represented with a single year. For example, an interval with the textual description “600 BC” can be described with a datetime description having a time:year value of -0599.

• periodo:earliestYear and periodo:latestYear for descriptions of intervals that need to be represented as ranges. For example, an interval with the textual description “eight century BC” can be described with a datetime description having a periodo:earliestYear value of -0799 and a periodo:latestYear value of -0700.

The datatype for values of time:year, periodo:earliestYear, and periodo:latestYear is xsd:gYear. Note that:

1. xsd:gYear values can have any number of digits.

2. xsd:gYear values may be zero. The value 0000 is interpreted as 1 BCE.

3. xsd:gYear values represent Gregorian calendar years and “are not, in general, convertible to simple values corresponding to years in other calendars.” We are comfortable with this limitation because we use these values only for the purposes of ordering and visualizing temporal extents of intervals. The skos:prefLabel of an interval should be considered the authoritative description.

We may use additional properties in our datetime descriptions in the future, for example to describe intervals at a finer temporal granularity than a year.

Spatial extent

We use the following properties to describe the spatial extent of periods:

• periodo:spatialCoverageDescription is used to textually describe the spatial extent exactly as given in the original source, for example “Near East and Greece”. The value of this property is a simple literal xsd:string, with no language tag.

• dcterms:spatial is used to link a period to descriptions of locations in gazetteers such as DBpedia/Wikidata, GeoNames, or Pleiades.