At first, historical linguistics served as the cornerstone of comparative linguistics primarily as a tool for linguistic reconstruction. Scholars were concerned chiefly with establishing language families and reconstructing prehistoric proto-languages, using the comparative method and internal reconstruction. The focus was initially on the well-known Indo-European languages, many of which had long written histories; the scholars also studied the Uralic languages, another European language family for which less early written material exists. Since then, there has been significant comparative linguistic work expanding outside of European languages as well, such as on the Austronesian languages and various families of Native American languages, among many others. Comparative linguistics is now, however, only a part of a more broadly conceived discipline of historical linguistics. For the Indo-European languages, comparative study is now a highly specialized field. Most research is being carried out on the subsequent development of these languages, in particular, the development of the modern standard varieties.
Some scholars have undertaken studies attempting to establish super-families, linking, for example, Indo-European, Uralic, and other families into Nostratic. These attempts have not been accepted widely. The information necessary to establish relatedness becomes less available as the time depth is increased. The time-depth of linguistic methods is limited due to chance word resemblances and variations between language groups, but a limit of around 10,000 years is often assumed. The dating of the various proto-languages is also difficult; several methods are available for dating, but only approximate results can be obtained.
Initially, all modern linguistics was historical in orientation. Even the study of modern dialects involved looking at their origins. Ferdinand de Saussure's distinction between synchronic and diachronic linguistics is fundamental to the present day organization of the discipline. Primacy is accorded to synchronic linguistics, and diachronic linguistics is defined as the study of successive synchronic stages. Saussure's clear demarcation, however, has had both defenders and critics.
In linguistics, a synchronic analysis is one that views linguistic phenomena only at a given time, usually the present, though a synchronic analysis of a historical language form is also possible. This may be distinguished from diachronic, which regards a phenomenon in terms of developments through time. Diachronic analysis is the main concern of historical linguistics; however, most other branches of linguistics are concerned with some form of synchronic analysis. The study of language change offers a valuable insight into the state of linguistic representation, and because all synchronic forms are the result of historically evolving diachronic changes, the ability to explain linguistic constructions necessitates a focus on diachronic processes.
In practice, a purely synchronic linguistics is not possible for any period before the invention of the gramophone, as written records always lag behind speech in reflecting linguistic developments. Written records are difficult to date accurately before the development of the modern title page. Often dating must rely on contextual historical evidence such as inscriptions, or, modern technology such as carbon dating can be used to ascertain dates of varying accuracy. Also, the work of sociolinguists on linguistic variation has shown synchronic states are not uniform: the speech habits of older and younger speakers differ in ways that point to language change. Synchronic variation is linguistic change in progress.
Synchronic and diachronic approaches can reach quite different conclusions. For example, a Germanic strong verb like English sing – sang – sung is irregular when viewed synchronically: the native speaker's brain processes these as learned forms, whereas the derived forms of regular verbs are processed quite differently, by the application of productive rules (for example, adding -ed to the basic form of a verb as in walk – walked). This is an insight of psycholinguistics, relevant also for language didactics, both of which are synchronic disciplines. However, a diachronic analysis will show that the strong verb is the remnant of a fully regular system of internal vowel changes, in this case, namely, the Indo-European ablaut; historical linguistics seldom uses the category "irregular verb".