the nineteenth letter of the english alphabet, is a consonant, and is often called a sibilant, in allusion to its hissing sound. it has two principal sounds; one a mere hissing, as in sack, this; the other a vocal hissing (the same as that of z), as in is, wise. besides these it sometimes has the sounds of sh and zh, as in sure, measure. it generally has its hissing sound at the beginning of words, but in the middle and at the end of words its sound is determined by usage. in a few words it is silent, as in isle, debris.  with the letter h it forms the digraph sh.  see guide to pronunciation, // 255-261.