Help:Basic membership properties
|This page in a nutshell:
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).
- 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.
- 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.
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.
- is used to state that all the instances of one class are instances of another
- more specifically, it is an rdfs:subClassOf
- human brain (Q492038) subclass of (P279) brain (Q1073)
- brain (Q1073) subclass of (P279) animal organ (Q24060765)
- animal organ (Q24060765) subclass of (P279) organ (Q712378)
- 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:
- 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).
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).
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.
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).
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
- 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.
|Property||X||Y||what it denotes||example||explanation||why not use…|
|<X> instance of <Y>||instance||class||
||<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)||
|<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||
|<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
|<hatter> instance of <profession>||<hatter> is a specific example of a profession||
|<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)
|<X> subclass of <Y>||class||class||
||<supercarrier> subclass of <aircraft carrier>||both <supercarrier> and <aircraft carrier> are classes and the latter contains the former||
|<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||
|<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>
|<X> part of <Y>||instance||instance||
||<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)||
|<People's Republic of China> part of <Asia>||both <China> and <Asia> are specific geographic features, <China> is part of the continent <Asia>||
|<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
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>)||
|<member state> part of <international organization>||an instance of <member state> is a component of an instance of <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||
|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||
|<Solar System> has part <Mars>||<Mars> is one of the planets in the <Solar System>||
|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||
|<galaxy> has part <star>||a <galaxy> has <stars> as one of its parts||
|<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.|
- 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.