Assimilation occurs when a sound comes to share one or more features with a neighboring sound. The neighboring sound may either appear to the left or to the right of the segment that is affected. When the affected sound precedes the one that causes the assimilation, we speak of regressive or anticipatory assimilation. An example is /n/ in ten books, which tends to become bilabial /m/ under the influence of the following bilabial /b/. If the sound that is assimilated follows the sound that it becomes more similar to, we speak of progressive or perseveratory assimilation. This occurs for example in disguise, where /g/ will be initially devoiced under the influence of the preceding voiceless /s/.
The assimilation may be complete or partial. Complete assimilation occurs when a sequence of two identical sounds is created, as in ten minutes or goodbye, where /n/ and /d/ may be replaced with /m/ and /b/, respectively. Such sequences are sometimes simplified so that only one sound is retained. The assimilation of /n/ in ten books is of course only partial, since the bilabial nasal /m/ still differs from the bilabial stop /b/.
What kind of features are involved in assimilations? Typically, the articulatory features that one segment may pass on to a neighboring one are: