Any language is a highly complex communication system. To understand the working principles of language, linguists distinguish different components or subsystems that give shape to this complexity. Thus, language can be described in terms of its sounds and sound patterns (phonetics & phonology), morphemes and words (morphology), grammatical word combinations (syntax), structures of meaning (semantics), and patterns of language use (pragmatics). However, language is not only structurally complex. Rather than consisting of relatively uniform and static components, language is a dynamic communication system that is subject to different dimensions of linguistic variation.

To tackle the complexity of the dynamics of language, linguists, in general, focus on three basic dimensions of linguistic variation:

1. social variation
2. spatial variation
3. temporal variation

The linguistic system varies relative to social factors, i.e. it shapes and is shaped by different dimensions of social organisation. In a nutshell, language varies depending on who speaks it to whom in which (social) context. The social variation and dynamics of language is focused at in the field of sociolinguistics. However, societies are themselves subject to variation and change with regard to cultural, political, or religious orientation. Thus, it should not come as a surprise that the language system(s) used by a generalised speech community are not static, but that they also vary through space and time.

Spatial variation within one broad-scale language system is usually dealt with in dialectology. Everybody who has ever left his or her home town and has travelled to another place - which does not even have to be very far away - is aware of this dimension of variation.

The historical dimension of language development and language change is the third basic dimension to describe the dynamics of linguistic structures and their corresponding conventions of usage. As a universal matter of fact, any language is subject to change. Historical linguistics is the field of linguistic research that has specialised in theanalysis of temporal variation.

It is important to stress the fact that that the social, historical, and spatial perspectives on linguistic variation are not mutually exclusive. Rather, they interact and co-shape each other in intriguing ways. For instance, language change is often triggered by social changes. Since social subgroups are often distributed in space (e.g. urban vs. rural life styles), such changes may also influence the spatial, i.e. dialectal, variation of language. Linguists must attempt to develop appropriate models to understand these complex relationships.

If, however, the stem of the plant is cut transversely, a very complicated design of rings appears. This design shows the complex arrangement of the tree’s fibers – its internal complex structure. By analogy, the tree rings can be compared to the complex structure of a language. In the same way as the transversal cut lays open the structure of the fibers at one stage in the tree’s growth, the overall structure of a linguistic system can be analysed at a given point in time. This is the synchronic perspective.

Historical linguistics deals with the complex interaction of the synchronic and diachronic perspectives on language. On the one hand, it can analyse textual (or other) sources from a given period and try to reconstruct the synchronic state of a given language at a specific point in time. Moving from one such point to the other, it becomes possible to describe the history of a language , for instance the history of English. On the other hand, historical linguistics is fundamentally interested in the principles according to which languages (and its subsystems) change. However, to be able to describe such general principles and processes of language change, historical linguists much compare different temporal states of the language in question. In short, the synchronic and diachronic dimensions of linguistic analysis are closely related. It is virtually impossible to understand one perspective without the other.

The historical dimension of linguistic variation places a temporal perspective on the dynamics of language. It is not controversial that English, for instance, has changed considerably since its earliest attested stage. The English language has undergone complex processes of reshaping and transformation and will do so in the future. Therefore, English historical linguistics is a very fascinating field of analysis indeed. But how can the historical dynamics of linguistic structure and use be approached scientifically?

The famous Swiss linguist Ferdinand de Saussure tried to tackle the two dimensions of structural complexity, on the one hand, and temporal change, on the other, by introducing the terms synchrony – or synchronic linguistics - and diachrony – or diachronic linguistics (see de Saussure 1915). To exemplify these two dimensions of linguistic analysis, de Saussure used a tree analogy. Trees evolve, develop, grow. In other words, they change their shape throughout their lifetime, without, however, changing their basic anatomy. To trace the development of a tree, one can observe its longitudinal growth from being a shoot to becoming a huge plant. This temporal perspective of evolution corresponds to the diachronic perspective on language.

Given the fact that any language changes, historical linguists are interested in the following questions related to this dynamism.

This perspective focuses on the complex dimensions of linguistic organisation: sound (phonetics, phonology), writing (graphology, orthography), inflection & word derivation (morphology), words (lexicology, lexical semantics), phrases & clauses (syntax), and the use of these structures (pragmatics).

Historical linguists try to describe the changes that these dimensions undergo throughout their historical development.

The changes in the subsystems described above do not occur at random but are triggered by a variety of causes and factors.

Historical linguists want to understand how these causes and factors provoke temporal variation.

The transformations in the linguistic system are not chaotic but subject to certain general principles and processes.

Historical linguists are interested in discovering the systematicity of language change to be able to make predictions about the past or future developments of a given language.

To pin down the history of a language such as English, it is necessary to focus on distinct periods of linguistic development. Although the diachronic transformation of a language is a gradual and continuous process, certain phases in this evolution are more stable than others.

Moreover, striking historical events often trigger quite radical changes in a linguistic system. Historical linguistics thus attempts to tell the history of a language relative to specific stages of development and phases of reshaping.