In articulatory phonetics, fortition, also known as strengthening, is a consonantal change that increases the degree of stricture.
Although not as typical of sound change as lenition, fortition may occur in prominent positions, such as at the beginning of a word or stressed syllable; as an effect of reducing markedness; or due to morphological leveling.
Fortition of the cross-linguistically rare interdental fricatives [θ] and [ð] to the almost universal corresponding stops [t] and [d] is relatively common.
In the Cushitic language Iraqw, *d has lenited to /r/ between vowels, but *r has undergone fortition to /d/ word initially.
In Bukusu, v [β] and w become b, y becomes j [dʒ], and l, r become d. In other languages, voiceless fricatives f, s, hl become affricates pf, ts, tl; see for example Xhosa.
[4] This is similar to the epenthetic stop in words like dance ([ˈdæns ~ ˈdænts]) in many dialects of English, which effectively is fortition of fricative [s] to affricate [ts].