In historical linguistics, a number of traditional terms designate types of phonetic change, either by nature or result. A number of such types are often (or usually) sporadic, that is, more or less accidents that happen to a specific form. Others affect a whole phonological system. Sound changes that affect a whole phonological system are also classified according to how they affect the overall shape of the system; see phonological change. Assimilation: One sound becomes more like another, or (much more rarely) two sounds become more like each other. Example: in Latin the prefix *kom- becomes con- before an apical stop () or : contactus "touched", condere "to found, establish", connūbium "legal marriage". The great majority of assimilations take place between contiguous segments, and the great majority involve the earlier sound becoming more like the later one (e.g. in connūbium, m- + n becomes -nn- rather than -mm-). Assimilation between contiguous segments are (diachronically speaking) exceptionless sound laws rather than sporadic, isolated changes. Dissimilation: The opposite of assimilation. One sound becomes less like another, or (much more rarely) two sounds become less like each other. Examples: Classical Latin quīnque "five" > Vulgar Latin *kinkʷe (whence French cinq, Italian cinque, etc.); Old Spanish omne "man" > Spanish hombre. The great majority of dissimilations involve segments that are not contiguous, but, as with assimilations, the great majority involve an earlier sound changing with reference to a later one. Dissimilation is usually a sporadic phenomenon, but Grassmann's Law (in Sanskrit and Greek) exemplifies a systematic dissimilation. If the change of a sequence of fricatives such that one becomes a stop is dissimilation, then such changes as Proto-Germanic *hs to (spelled x) in English would count as a regular sound law: PGmc. *sehs "six" > Old English siex, etc. Metathesis: Two sounds switch places. Example: Old English thridda became Middle English third. Most such changes are sporadic, but occasionally a sound law is involved, as Romance *tl > Spanish ld, thus *kapitlu, *titlu "chapter (of a cathedral)", "tittle" > Spanish cabildo, tilde. Metathesis can take place between non-contiguous segments, as Greek amélgō "I milk" > Modern Greek armégō. Lenition, softening of a consonant, e.g. stop consonant to affricate or fricative; and its antonym fortition, hardening of a consonant. Tonogenesis: Syllables come to have distinctive pitch contours. Sandhi: conditioned changes that take place at word-boundaries but not elsewhere. It can be morpheme-specific, as in the