William Stewart, in 1965, proposed the terms acrolect, the highest or most prestigious variety on the continuum, and basilect, the lowest or least prestigious variety, as sociolinguistic labels for the upper and lower boundaries, respectively, of a post-creole speech continuum. In the early 1970s Derek Bickerton popularized these terms (as well as mesolect for intermediate points in the continuum) to refer to the phenomenon of code-switching used by some users of creole languages who also have some fluency in the standard language upon which the contact language is based. University of Chicago linguist Salikoko Mufwene explains the phenomenon of creole languages as "basilectalization" away from a standard, often European, language among a mixed European and non-European population. In certain speech communities, a continuum exists between speakers of a creole language and a related standard language. There are no discrete boundaries between the different varieties and the situation in which such a continuum exists involves considerable social stratification.