⟨ff⟩ is used in English orthography for the same sound as single ⟨f⟩, /f/. The doubling is used to indicate that the preceding vowel is (historically) short, or for etymological reasons, in latinisms. Very rarely, ⟨ff⟩ may be found word-initially, such as in proper names (e.g. Rose ffrench, Jasper Fforde). In the Welsh alphabet, ⟨ff⟩ represents /f/, while ⟨f⟩ represents /v/. In Welsh, ⟨ff⟩ is considered a distinct letter, and placed between ⟨f⟩ and ⟨g⟩ in alphabetical order. In medieval Breton, vowel nasalisation was represented by a following ⟨ff⟩. This notation was reformed during the 18th century, though proper names retain the former convention, which leads to occasional mispronunciation.
⟨fh⟩ is used in Irish orthography for the lenition of ⟨f⟩. This happens to be silent, so that ⟨fh⟩ in Irish corresponds to no sound at all. For example, the phrase cá fhad ('how long') is pronounced, where fhad is the lenited form of fad ('long').
⟨fx⟩ in used in the orthography of Nambikwara for a glottalized /ɸʔ/.
Read more about this topic: List Of Digraphs In Latin Alphabets
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