Compounds in which an anionic site Y⁻ (originally on carbon, but now including other atoms) is attached directly to a heteroatom X⁺ (usually nitrogen, phosphorus or sulfur) carrying a formal positive charge. They are thus 1,2-dipolar species of the type R_mX⁺–Y⁻R_n. If X is a saturated atom of an element from the first row of the periodic system, the ylide is commonly represented by a charge-separated form; if X is a second, third, etc. row element uncharged canonical forms are available R_mX=YR_n. If X is an unsaturated atom, doubly bonded to another first row element Z, the negative charge on Y may be stabilized by π-@C01267@, Z=X⁺–Y⁻R_n\(\leftrightarrow\) Z⁻–X⁺=YR_n. Such ylides belong to the class 1,3 @D01753@. However, 1,3-@D01753@ with only sextet-containing canonical forms (e.g. @V06620@) are not ylides. E.g. Ph₃P⁺–C⁻H₂\(\leftrightarrow\) Ph₃P⁺=CH₂ (often called a Wittig @R05190@), (CH₃)₃N⁺–C⁻H₂, RC≡N⁺N⁻–R, (CH₃)₂S=CHPh\(\leftrightarrow\) (CH₃)₂S⁺–C⁻HPh. Note that ylide is a complete word, not to be confused with the suffix -ylide, used for some radical anions. Subclasses of ylides: Ylides R_mX⁺–C⁻R₂ having the negative charge on carbon are classified by citing the name of the element X before the word ylide. E.g. nitrogen ylide, @P04563-1@, @O04379@, sulfur ylide. A further specification may be achieved by citing the class name of R_mX before the word ylide. Thus nitrogen ylides include @A00275@, R₃N⁺–C⁻R₂, @A00565@ R₂C=N⁺R–C⁻R₂, @N04153@, RC≡N⁺–C⁻R₂. Some authors, who wish to express the positive charge on X, prefer e.g. @A00292@ over @A00275@; such usage varies according to the heteroatom X and to national custom. The ylides R_mX⁺–Y⁻\(\leftrightarrow\) R_mX=Y (Y = O, S, Se, Te, NR) are usually named by citing the name of R_mX followed by the @A00134@ nomenclature term for Y (oxide, sulfide, selenide, telluride, imide, respectively). E.g. @A00271@; use of the less systematic synonyms @A00272@ and aminimines is discouraged. Some classes of ylides are known by trivial names e.g. @N04164@, @P04563-2@ (synonymous with @P04563-3@).