Although certain changes constantly occur at one or another linguistic level, the historical development of language cannot be regarded as permanent instability. Many features of the language remain static in diachrony: these constant features do not alter through time or may be subject to very slight alteration.
In the first place there exist certain permanent, universal properties to be found in all languages at any period of time, such as e.g. the division of sounds into vowels and consonants, the distinction between the main parts of speech and the parts of the sentence. In addition to these universal properties, English, like other languages, has many stable characteristics which have proved almost immune to the impact of time. For instance, some parts of the English vocabulary have been preserved through ages; to this stable part belong most of the pronouns, many form-words and words indicating the basic concepts of life. Many ways of word-formation have remained historically stable. Some grammatical categories, e.g. number in nouns, degrees of comparison in adjectives, have suffered little alteration while other categories, such as case or gender, have undergone profound changes. The proportion of stable and changeable features varies at different historical periods and at different linguistic levels but there is no doubt that we can find statics and dynamics both in synchrony and in diachrony. Dynamics in diachrony, that is linguistic change, requires special consideration.
