It should be mentioned here, however, that like all other divisions of history into periods, this division of the history of the English language into certain periods is only a matter of procedural convenience. Most of the changes that take place in a language are of a slow and continuous nature, spreading over long stretches of time.
History, particularly the history of a language, is a continuous interplay of two opposite tendencies: the urge to innovate and the urge to conserve. At times one of the two tendencies asserts itself much more forcefully than the other but both the tendencies are basic, instinctual and always in operation.
It would be naive, therefore, to conceptualize these periods as neat blocks of time with sharply defined edges, with a definite month or year to mark the beginning of any one of the periods and with another definite month or year to mark its end.
Every period shades off into the next, gradually and imperceptibly, like a leaf coming out of a tree. The dates mentioned here are, therefore, not sharp lines of division; they are only convenient cut-off points of an arbitrary nature introduced to facilitate our study of the subject-matter.