Help:Basic membership properties

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This page in a nutshell:

Instances are individuals or single things

Classes are abstractions (that might or might not have instances at Wikidata)

Wikidata items are instances and/or classes

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 abstract object that represents a set of items, called its instances. Typically, all the instances belonging to a class each share a set of properties, which properties characterize the class. The instances differ from each other in the values they have for those properties, but not in the fact of having the properties themselves. Thus, each class is typically characterized by the properties that all of its instances share (although this is not enforced by Wikidata).

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 house cat (Q146) with instance Grumpy Cat (Q7714263), etc.
class election (Q40231) with instance 2016 Taiwan presidential election (Q20683626), etc.

Practical aspects

Nothing prevents an item from being both an instance and a class. An item is an instance simply by virtue of having an instance of (P31) property. Similarly, an item is a class simply by virtue of being the value associated with some other item's instance of (P31) property (or by having a subclass property).
The complete set of instances implied by a class might or might not exist as items within Wikidata.
If, from a logical perspective, all the items conceptually belonging to class A (whether or not they are actually instantiated in Wikidata) must also belong to class B, then the relation between A and B is called subclass of. We say that A is a subclass of B.

instance of (P31)

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:

subclass of (P279)

is used to state that all the instances of one class are instances of another^[2]
more specifically, it is an rdfs:subClassOf^[3]

Several examples:

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 is 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:

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).

subclass of (P279) is transitive property (Q18647515), that means if an item A is an instance of class B, and class B is a subclass of class C, item A is implicitly also an instance of class C. There is no general need to add a statement for the relation A→C to Wikidata.

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).

part of (P361)

Items can not be 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.

Albert Einstein's brain (Q2464312) part of (P361) Albert Einstein (Q937).

Classes can also be part of another class. For example,

human brain (Q492038) part of (P361) Homo sapiens (Q15978631).

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 (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 parts of the class (P2670) - is used to say that an instance has as part an instance of a class

For example,

Greek (Q8216) has part (P527) Α (Q9887) (instance-instance)
alphabet (Q9779) has part (P527) letter (Q9788) (class-class)
Greek (Q8216) has parts of the class (P2670) Greek letter (Q19793459) (instance-class)

In the first example, has part (P527) is used because Greek (Q8216) has as part Α (Q9887). has part (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 parts of the class (P2670) since Greek (Q8216) has one or more instances of Greek letter (Q19793459) as parts.

Examples

Differences among instance of (P31), subclass of (P279) and part of (P361)
Property	X	Y	what it denotes	example	explanation	why not use…
<X> instance of <Y>	instance	class	X is an individual (single thing) belonging to class Y X is a specific example of class Y all instances of class Y share certain features Y is the set of its instances (including X)	<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)	subclass of not used because <USS Nimitz> is not a class part of not used because <USS Nimitz> is an instance and the <supercarrier> is a class, instances cannot be parts of classes
				<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	subclass of not used because <China> is a concrete state part of not used because instances cannot be parts of classes
				<Sol> 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 part of not used because non-classes cannot be parts of classes
				<hatter> instance of <profession>	<hatter> is a specific example of a profession	subclass of not used because <hatter> is not a class of professions but a single profession (instance) part of not used because instances cannot be parts of classes
				<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) part of not used because instances cannot be parts of classes
<X> subclass of <Y>	class	class	all instances of class X belong to class Y instances of X are also instances of Y	<supercarrier> subclass of <aircraft carrier>	both <supercarrier> and <aircraft carrier> are classes and the latter contains the former	instance of not used because <supercarrier> is not a concrete object part of not used because an instance of <supercarrier> (e.g. <USS Nimitz>) cannot be part of an instance of aircraft carrier (e.g. <USS Enterprise>)
				<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	instance of not used because <sovereign state> is not a specific state, but a class that contains all sovereign states part of not used because an instance of the sovereign state (e.g. <China>) cannot be part of an instance of state (e.g. <China>)
				<G-type stars> 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> part of not used because the relationship is not one of composition, i.e. individual <G-class star> instances do not join together to make an instance of a <star>.
<X> part of <Y>	instance	instance	instance X is a part of instance Y different parts of Y (including X) combine together to form the whole Y	<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)	instance of not used because <CSG-11> is not a class subclass of not used because neither of them is a class
				<People's Republic of China> part of <Asia>	both <China> and <Asia> are specific geographic features, <China> is part of the continent <Asia>	instance of not used because <Asia> is an instance (meanwhile <Asian country> is a class so you could say <China> is an instance of <Asian country>) subclass of not used because neither of them is a class
				<Sol> 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 subclass of not used because neither of them is a class
	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>)	instance of not used because <flight deck> is a class not an instance (only a specific flight deck, such as the <flight deck of USS Nimitz>, can be an instance) subclass of not used because an instance of the former (e.g. <flight deck of USS Nimitz>) cannot be the same thing as an instance of the latter (e.g. <USS Nimitz>)
				<member state> part of <international organization>	an instance of <member state> is a component of an instance of <international organization>	instance of not used because the former is a concept, does not indicate any specific state (e.g. <China>) subclass of not used because an instance of <member state> does not belong to the class <international organization> (e.g. China is not an international organization)
				<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	instance of not used because <star> is not an example of a <galaxy> subclass of not used because instances of class <star> are not instances of the class <galaxy>
Note: some examples above are for demonstration purposes only, which should not be used in Wikidata because there are more specific relationships that should be used (e.g. use "<China> continent <Asia>" instead of "<China> part of <Asia>").

Differences among has part (P527) and has parts of the class (P2670)
Property	X	Y	what it denotes	example	explanation	why not use…
<X> has part <Y>	instance	instance	instance X has instance Y among its parts or components	<United States Congress> has part <United States Senate>	the US Congress has two parts, the House of Representatives and the Senate	has parts of the class not used because <United States Congress> and <United States Senate> are concrete objects
	instance	instance	instance X has instance Y among its parts or components	<Solar System> has part <Mars>	<Mars> is one of the planets in the <Solar System>	has parts of the class not used because <Solar System> and <Mars> are concrete objects
	class	class	an instance of X has an instance of Y among its parts or components	<body> has part <head>	in general a <body> (anatomical feature) has a <head> as one of its parts	has parts of the class not used because <body> and <head> are both classes
	class	class		<galaxy> has part <star>	a <galaxy> has <stars> as one of its parts	has parts of the class not used because <galaxy> and <star> are both classes
<X> has parts 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 parts of the class <college of the University of Cambridge>	the <University of Cambridge> has colleges as parts	* has part not used because <University of Cambridge> does not have the class <college of the University of Cambridge> as a part, but instead has colleges as parts
				<Solar System> has parts of the class <inner planet>	our <Solar System> has one or more <inner planets> in its parts	* has part not used because <Solar System> does not have the class of <inner planets> as a part, but instead has planets as parts
				<Albert Einstein> has parts of the class <human brain>	<Albert Einstein> had a <human brain> as a part	* has part not used because <human brain> is a class, not an instance, even if he had just one.

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References

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)

[1] ttps://www.w3.org/TR/rdf-schema/#ch_type

[2] ttps://www.w3.org/TR/rdf-schema/#ch_subclassof

[3] ttps://www.w3.org/TR/rdf-schema/#ch_subclassof

[1]

[2]

[3]