The exception to this rule is when a common “lingua franca” has evolved to facilitate communication between different linguistic groups. This has been the case for Classical Nahuatl and Classical Maya, both of which, at different times in history, have been used as a common language between different ethnic groups. Further complicating matters are the semi-nomadic lifestyle of many Mesoamerican peoples, and political systems which often have used relocation of entire communities as a political tool. Dialect or variant “chaining” is common, where any adjacent two or three towns in a sequence are similar enough in speech to understand each other fairly well, but those separated more widely have trouble understanding each other, and there are no clear breaks naturally separating the continuum into coherent sub-regions. All of these factors together have made it exceedingly difficult to distinguish between what constitutes a language or a dialect in Mesoamerica. Linguistic isoglosses do not coincide often or strongly enough to prove very useful when trying to decide, and sociological factors often further cloud the picture. The significance of measurements of intelligibility (which is itself difficult to measure) depends very much on analysts' purposes and theoretical commitments. In Spanish the word has often been used generically about indigenous languages in order to describe them as inherently inferior to the European languages. In recent years this has caused an aversion to the term “dialect” among Spanish-speaking linguists and others, and the term has often been applied instead. Many Mesoamerican linguistic groupings have not had different names in common usage for their different languages and some linguistic groups known by a single name show a sufficiently significant variation to warrant division into a number of languages which are quite low in mutual intelligibility. This is the case for example for the Mixtecan, Zapotecan and Nahuan linguistic groups, which all contain distinct languages that are nonetheless referred to by a single name. Sometimes a single name has even been used to describe completely unrelated linguistic groups, as is the case with the terms "Popoluca" or "Chichimeca". This shortage of language names has meant that the convention within Mesoamerican linguistics when writing about a specific linguistic variety is to always mention the name of the broad linguistic group as well as the name of the community, or geographic location in which it is spoken, for example Isthmus-Mecayapan Nahuatl, Zoogocho Zapotec or Usila Chinantec. Some language groups however