Artificial languages—languages which have been consciously designed—have been created for more than 900 years, although the number of them has increased considerably in recent decades, and by the early 21st century the total figure probably was in the thousands. There have been several goals behind their creation; the traditional one (which applies to some of the best-known artificial languages, including Esperanto) is to make international communication easier. Some other well-known artificial languages, such as Klingon, have been designed in connection with works of fiction. Still others are simply personal projects. A traditional way of classifying artificial languages involves the extent to which they make use of material from natural languages. Those artificial languages which are created mainly by taking material from one or more natural languages are called a posteriori languages (which again include well-known languages such as Esperanto), while those which do not use natural languages as sources are a priori languages (although many a posteriori languages have a limited amount of a priori material, and some a priori languages have a small number of a posteriori components). Between these two extremes are the mixed languages, which have large amounts of both a priori and a posteriori material. Artificial languages can also be classified typologically (as natural languages are) and by how and how much they have been used. Many linguists seem to be biased against research on artificial languages, although some major linguists of the past have been interested in them.