A consonant is a speech sound - not a letter - caused by a constriction or closure at some point along the vocal tract. Every syllable is understood to also contain a vowel sound but does not have to include a consonant.
Most English dialects have 24 consonants (counting semivowels /j/ and /w/). Many consonants are normally spelled with one letter or digraph, but several of them have an inconsistent spelling. For example /ʃ/ can unambiguously be spelled as "sh", but there are several other spellings such as "ti" in nation or "ce" in ocean.
The following letters or digraphs unambiguously spell consonant phonemes: b, ch, d, g, h, j, k, l, m, n, p, r, sh, t, v, w, y, z.
The following phonemes can't be spelled unambiguously:
- /θ/ as in thank, because "th" can be either /θ/ or /ð/.
- /ð/ as in then, for the same reason
- /ŋ/ as in thing, because "ng" can represent /ŋ/ or /ŋg/ (as in "finger")
- /ʒ/ as in pleasure (however "zh" is sometimes used).