Synchrony and diachrony are two opposite perspectives taken on linguistic phenomena. In the diagram, the vertical axis is the time axis.

• Synchrony is the relation of a linguistic phenomenon to phenomena existing simultaneously in the language system. It concerns a stage of a language and implies, with respect to the evolution of the language, a static view of it.¹
• Diachrony is the relation of a linguistic phenomenon to such phenomena which correspond to it in a preceding or following stage of the language. It concerns linguistic change and implies, with respect to the evolution of the language, a dynamic view of it.

Both of these concepts are commonly misunderstood in various ways:

• There are no synchronic or diachronic phenomena and, consequently, no phenomenon that could only be described or explained by synchronic or by diachronic linguistics.
• Neither of the two perspectives has priority over the other; they complement each other.²
• It is not the case that a linguistic description has to be either purely synchronic or purely diachronic. On the contrary, given that the two perspectives complement each other, a linguistic phenomenon can be completely understood only under both perspectives taken together. What is necessary is that the two perspectives be kept cleanly apart in the methodology.
• Synchrony is not the examination of a language in its contemporary state. There are synchronic descriptions of ancient states of a language.
• Likewise, diachrony is not necessarily concerned with the past.³ An investigation prognosticating the future evolution of a language is a diachronic one.
• Diachrony is not the same as history:
  • A historical examination of a linguistic phenomenon presupposes that there be documents attesting it; a diachronic examination only requires a temporal dynamism. Consequently, prehistorical – especially reconstructed – phenomena may be viewed in diachronic perspective, but by definition not in a historical perspective.
  • Diachrony is – just like synchrony – a systematic perspective viewing an object in a certain respect. In linguistics, this object is generally the linguistic system or parts of it.⁴ By contrast, language history does not limit itself to the history of the linguistic system, but also comprises the so-called external history of the language.
  • A historical examination relates a phenomenon to its social, cultural, political etc. context, thus trying to understand it as historically conditioned and possibly unique. A diachronic examination does nothing of the kind, but instead searches some dynamism internal to the language system that the phenomenon is subject to. There are diachronic laws, but no historical laws.

The confusion between diachrony and history is extremely common in linguistics, but practically not found outside it. This is obviously because linguistics – differing in this from other disciplines interested in history – looks for generalizations and many linguists are actually more interested in these than in historical facts.⁵

¹ It implies neither a static nor a dynamic view of its system at a given stage of its evolution.

² The idea of the priority of synchrony is apparently due to Ferdinand de Saussure (1916). It presupposes a fundamentally static view of language. Language had already been conceived as an activity by Wilhelm von Humboldt (1836). The language system is the product of the constant systematizing activity of its speakers (Coseriu 1958).

³ Analogous considerations apply to historical phenomena. Especially the English word historical implies, in informal use, “related to former times”. Instead, a historical phenomenon is one bound up with time and culture (as opposed to a natural phenomenon). Such phenomena always exist, even today and tomorrow.

⁴ On the grounds of the diverse relations constituting the above two definitions, the language system is usually conceived as such only in synchronic perspective. However, this is just a side-effect of the condition that it is very complicated to conceive and describe the diachrony of entire linguistic systems. On the other hand, very few synchronic studies indeed account for an entire language system.

⁵ Moreover, in anglophone linguistics the adjective historical is frequently used instead of diachronic. For instance, in 2022 The Cambridge Handbook of Historical Syntax is published. Such a handbook is already difficult to imagine in purely conceptual terms. The table of contents then provides complete clarity that it is a handbook on diachronic syntax.