Help:Basic membership properties
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Introduction
This page explains the differences among the properties instance of (P31), subclass of (P279) and part of (P361).
Definitions
To understand the differences, it is important to be familiar with the terms instance and class. A class is an abstraction that describes a set (or collection) of things, called its instances. Typically, all the instances belonging to a class share a set of properties, which properties characterize the class.
If all the instances of class A necessarily also belong to class B, then we say A is a subclass of B.
Examples
- class human (Q5) with instances Abraham Lincoln (Q91), Mahatma Gandhi (Q1001), Isaac Newton (Q935), etc.
- class lighthouse (Q39715) with instances Amrum Lighthouse (Q20669), Lighthouse of Alexandria (Q43244), etc.
- class ocean (Q9430) with instances Atlantic Ocean (Q97), Pacific Ocean (Q98), etc.
- class public election (Q40231) with instance 2016 Taiwanese presidential election (Q20683626), etc.
Practical aspects
- An item can be both an instance and a class. An item is an instance if it has an instance of (P31) property. An item is a class if it is the value associated with some other item's instance of (P31) property (or if it has a subclass of (P279) property).
- The complete set of instances implied by a class might or might not exist as items within Wikidata.
- Typically, a class is characterized by a set of properties shared by all instances. The instances differ from each other in the values they have for those properties, but not in the fact of having the properties themselves (although this is not enforced by Wikidata).
The relation between instances with a common feature and a class characterized by this feature is produced with the property instance of (P31). We use instance of (P31) instead of subclass of (P279) when we cannot say anything about instances with such relation. More specifically, it is an rdf:type.[1]
For example, Atlantic Ocean (Q97) and Pacific Ocean (Q98) are both instances of ocean (Q9430). We write therefore on Wikidata:
- Is used to state that all the instances of one class are instances of another[2]
- More specifically, it is an rdfs:subClassOf[3]
- If any instances of the subject class are not instances of the value class, subclass of (P279) is not appropriate. partially coincident with (P1382) may be used to link overlapping classes where neither is a strict subclass of the other.
Several examples:
- human brain (Q492038) subclass of (P279) brain (Q1073)
- brain (Q1073) subclass of (P279) animal organ (Q24060765)
- animal organ (Q24060765) subclass of (P279) organ (Q712378)
Another example:
- class lake (Q23397) with instances Lake Baikal (Q5513) and Lake Erie (Q5492);
- class ocean (Q9430) with instances Atlantic Ocean (Q97) and Pacific Ocean (Q98).
A lake and an ocean are not the same but all their instances share the common feature of being a body of water. Therefore we can use the class body of water (Q15324) to state that:
- lake (Q23397) subclass of (P279) body of water (Q15324);
- ocean (Q9430) subclass of (P279) body of water (Q15324).
Now Lake Baikal (Q5513), Lake Erie (Q5492), Atlantic Ocean (Q97) and Pacific Ocean (Q98) will all be transitive (indirect) instances of body of water (Q15324). In general we can use more abstract objects (like body of water (Q15324)) instead of enumerations (lake (Q23397) and ocean (Q9430)) both: 1. in our statements and 2. in our questions (not covered at this help page).
Transitive properties
The property subclass of (P279) is a transitive Wikidata property (Q18647515). This means: if a class A is a subclass of class B, and B is a subclass of class C, then A is implicitly also a subclass of C. There is then no general need to add an explicit statement to Wikidata declaring A to be a subclass of C.
For example, tree (Q10884) is a subclass of woody plant (Q757163), and woody plant (Q757163) is a subclass of plant (Q756). Thus tree (Q10884) is implicitly also a subclass of plant (Q756).
Similarly, if item x is an instance of class B, and B is a subclass of class C, then x is implicitly also an instance of C. There is then no general need to add an explicit statement to Wikidata declaring x to be an instance of C.
For example, Lighthouse of Alexandria (Q43244) is an instance of lighthouse (Q39715) and lighthouse (Q39715) is a subclass of tower (Q12518). Lighthouse of Alexandria (Q43244) is thus an instance of tower (Q12518).
The property instance of (P31) is not transitive. Considering the example from above: Angela Merkel (Q567) is an instance of politician (Q82955), and politician (Q82955) is an instance of profession (Q28640), but it would be incorrect to say that Angela Merkel (Q567) is an instance of profession (Q28640).
Inferences based on transitivity do not take qualifiers into account, so instance of (P31) and subclass of (P279) statements must be valid while ignoring any qualifiers; in other words, these statements should not have any restrictive qualifier (Q61719275), or any qualifier that is being used restrictively. For example, if A is often, but not always, a subclass of B, then it is not valid to state that A is a subclass of B with nature of statement (P5102) = often (Q28962312), because this will lead to false inferences.
Items cannot be an instance of another non-class, but they can be part of another non-class. For example, Albert Einstein's brain (Q2464312) is a part of Albert Einstein (Q937). On Wikidata we use part of (P361) for this relation.
Classes can also be part of another class. For example,
The use of instance of (P31) instead of part of (P361) would be wrong here because human brain (Q492038) is not a person. Using subclass of (P279) would also be wrong because an instance of human brain (Q492038) (e.g. Albert Einstein's brain (Q2464312)) is not an instance of the class Homo sapiens (Q15978631).
Like subclass of (P279), part of (P361) is a transitive property.
Inverse relations of part of (P361)
- has part(s) (P527) - is used to say that an instance has as part some other instance or that instances of a class have as part an instance of some other class
- has part(s) of the class (P2670) - is used to say that an instance (never a class) has as part an instance of a class. If the parent item is both an instance and a class, has part(s) of the class (P2670) should not be used.
For example,
- Greek alphabet (Q8216) has part(s) (P527) Α (Q9887) (instance-instance)
- alphabet (Q9779) has part(s) (P527) letter (Q9788) (class-class)
- Greek alphabet (Q8216) has part(s) of the class (P2670) Greek letter (Q19793459) (instance-class)
In the first example, has part(s) (P527) is used because Greek alphabet (Q8216) has as part Α (Q9887). has part(s) (P527) is also used in the second example because instances of alphabet (Q9779) have instances of letter (Q9788) as parts. In the last example, we take has part(s) of the class (P2670) since Greek alphabet (Q8216) has one or more instances of Greek letter (Q19793459) as parts.
Examples
| Property | X | Y | what it denotes | example | explanation | why not use… |
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| <X> instance of <Y> | instance | class |
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<USS Nimitz> instance of <supercarrier> | <USS Nimitz> is a single concrete aircraft carrier, <supercarrier> is an aircraft carrier class which has many instances (aircraft carriers) |
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| <People's Republic of China> instance of <sovereign state> | <sovereign state> is a concept defined by some features, <China> is an object which meet these features |
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| <Sun> instance of <G-type main-sequence star> | The <Sun> is a specific star with the spectral characteristics of a G-type main-sequence star, and so is an instance of that class | * subclass of not used because the <Sun> is not a class but a single astronomical object
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| <hatter> instance of <profession> | <hatter> is a specific example of a profession |
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| <Douglas Adams> instance of <human> | <Douglas Adams> is a specific human person (this is one of the most common cases for instance of) | * subclass of not used because <Douglas Adams> is not a class of people but a single person (instance)
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| <X> subclass of <Y> | class | class |
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<supercarrier> subclass of <aircraft carrier> | both <supercarrier> and <aircraft carrier> are classes and the latter contains the former |
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| <sovereign state> subclass of <state> | both of them are classes, the former has all features of the latter and some additional features, so it is a subclass of the latter |
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| <G-type star> subclass of <star> | every individual G-class star (instances of <G-class star>) is also a star - i.e. belongs to the class <star> | * instance of not used because <G-class star> is not a single star, but a type of <star>
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| <X> part of <Y> | instance | instance |
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<USS Nimitz> part of <Carrier Strike Group Eleven> | <USS Nimitz> is a concrete aircraft carrier, <CSG-11> is a concrete carrier strike group, <USS Nimitz> is one of <CSG-11>'s components (its flagship) |
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| <People's Republic of China> part of <Asia> | both <China> and <Asia> are specific geographic features, <China> is part of the continent <Asia> |
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| <Sun> part of <Solar System> | both of them are individual astronomical objects; the solar system is composed of the Sun, planets, and other objects in the Sun's vicinity | * instance of not used because the latter is an individual astronomical object, not a generic concept
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| class | class | an instance of class X is part of an instance of class Y | <flight deck> part of <aircraft carrier> | every aircraft carrier (instance of <aircraft carrier>) has its own flight deck (instance of <flight deck>) |
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| <member state> part of <international organization> | an instance of <member state> is a component of an instance of <international organization> |
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| <star> part of <galaxy> | a star typically is born and lives within a single galaxy which is made up of many stars and other astronomical objects |
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Here is a table that explains the difference between has part(s) (P527) and has part(s) of the class (P2670).
has part(s) (P527) should be used for class-class and instance-instance relationships.
has part(s) of the class (P2670) should be used for instance-class relationships. If you see a constraint violation on an item with has part(s) of the class (P2670)...
- If the item is a class and has subclass of (P279), it should use has part(s) (P527) instead of has part(s) of the class (P2670), even if it also has instance of (P31).
- The item may wrongly be a class and should instead be an instance. Remove subclass of (P279) and make sure it has a proper instance of (P31).
| Property | X | Y | what it denotes | example | explanation | why not use… |
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| <X> has part(s) <Y> | instance | instance | instance X has instance Y among its parts or components | <United States Congress> has part(s) <United States Senate> | the US Congress has two parts, the House of Representatives and the Senate |
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| <Solar System> has part(s) <Mars> | <Mars> is one of the planets in the <Solar System> |
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| class | class | an instance of X has an instance of Y among its parts or components | <body> has part(s) <head> | in general a <body> (anatomical feature) has a <head> as one of its parts |
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| <galaxy> has part(s) <star> | a <galaxy> has <stars> as one of its parts |
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| <X> has part(s) of the class <Y> | instance | class | the specific item X include some instance of class Y among its parts or components | <University of Cambridge> has part(s) of the class <college of the University of Cambridge> | the <University of Cambridge> has colleges as parts |
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| <Solar System> has part(s) of the class <inner planet of the Solar System> | our <Solar System> has one or more <inner planets> in its parts |
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| <Albert Einstein> has part(s) of the class <human brain> | <Albert Einstein> had a <human brain> as a part |
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Queries
Wikidata Queries can be used to find members of a given class, for example all subclasses of "Literary Work"; a Wikidata Query Builder is available to help.
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See also
References
- ↑ https://www.w3.org/TR/rdf-schema/#ch_type
- ↑ https://www.w3.org/TR/rdf-schema/#ch_subclassof
- ↑ https://www.w3.org/TR/rdf-schema/#ch_subclassof
External links
- Multi-Level Conceptual Modeling: Theory and Applications — explains differences between instance of (P31) and subclass of (P279) under the conceptual framework of multi-level conceptual modelling. Skip to part 4 for Wikidata-related content.
- Collections and Individuals — explains differences between instance of (P31) and subclass of (P279) (in the context of the Cyc (Q1147294) knowledge base)