a, b, c, ch, d, dd, e, f, ff, g, ng ,h, i, l, ll, m, n, o, p, ph, r, rh, s, t, th, u, w, y.
The letter "J" is slowly coming into the Welsh language.
While the letters ch, dd, ff, ng, ll, ph, rh, th are each written with two symbols (they are technically called digraphs), they are all considered to be single letters. This means, for example that Llanelli (a town in South Wales) has only 6 letters when read in Welsh, compared to 8 letters in English. Consequently, they each take up only a single space in Welsh crosswords.
Sorting is done in correspondence with the alphabet. For example, la comes before lz, which comes before lla, which comes before ma.
See also: Alphabet, Welsh language