When we look at classical languages, such as Ancient Greek, there is often an initial tendency to think of them as fixed systems that show little variation over time. This is probably a consequence of the fact that we only encounter Ancient Greek in the written form. Consider a modern language, like English. It is extremely difficult to be aware of the changes in usage taking place within your own lifetime—the way you used to say things versus how you say them now. However, if you examine the English used in earlier periods, it becomes clear that the language has undergone significant change over time. Works like Chaucer's Canterbury Tales, for example, are now extremely difficult to read without intensive study. So, if you consider a large-enough time frame, it becomes clear that the English language, even in written form, has gone through profound changes. It's much easier to observe changes in spoken language—you only have to consider the difference in speech patterns between older and younger generations of the same family. Often, the language used by teenagers is unintelligible to their grandparents, for example. Therefore, it's fundamental to differentiate between spoken and written forms when thinking about a language's evolution; while written language tends to be more conservative due to the influence of institutions and traditions, speech is less easily monitored and is constantly changing. And, studies into Ancient Greek demonstrate that it was subject to the same variation.