Scottish National Dictionary (1700–)
A, letter of alphabet.
I. In the Latin alphabet, on which our alphabet is founded, A, a, had the value of the vowel in English hard and in the first syllable of English father. The letter A, a, in Scots spelling stands for the following sounds (for value of symbols see Intro. pp. xlii-xlv):
1. [ɑ] Ex. caddie, haddie, sattle, brattle (noise), sappie (juicy), falla (fellow), marro, chafts (jaws), lat (let), dag (drizzle), thack (thatch).
2. [ɑ:] Ex. (1) twa, wha; (2) ba (ball), bla (blow), ca (call), wa (wall). In (2) the consonant l or w has been vocalised and then absorbed by the preceding a, resulting in a long vowel sound the same as a in English father. The loss of l or w in these words is often marked by an apostrophe — e.g. ba'.
3. [e] Same sound as in the Scottish educated pronunciation of fate, Fr. fé. This sound occurs when a is followed by a consonant + a vowel, generally e. Ex. bane (bone), pape (pip), sape (soap), gane (gone), rade (rode), rape (rope), wale (choose).
4.  In some districts a broader sound may be heard for [e] nearly equal to the vowel heard in the Sth.Eng. pronunciation of hair, wear. (See Intro. p. xliii.)
5. [e1] This sound is used instead of [e] in some dialects — e.g. Ags. See Intro. § 118.1. Ex. lape, v., mare, spale, grane, travis.
6. [è] This is another variety of [e] heard in some dialects — e.g. s.Sc. and mn.Sc. In the latter it occurs before the consonants p, t, k, b, d, l, m, n.
II. A forms a digraph, but not a diphthong, along with itself and other vowel symbols, i, y, u, and the consonant w.
1. It is joined with itself to indicate a long vowel [ɑ:]. This spelling is common in I.Sc., n.Sc. and s.Arg. as a substitute for a and au. Ex. aa (all), aave (scummer), faar (where), taapie (a soft, stupid, gawky person).
2. It is joined with i or y, the latter being used generally in final position, to denote [e], [e1], [è]. Ex. rair (roar), airmy, laits (morals), gait (a goat), quait (quiet), flay (frighten).
3. It is joined with u to indicate (1) [ɑ:], (2) the sound , which generally displaces aa (see II. 1) in central Scots. Until recently  was not known in s.Ayr, Galloway, Dmf., Rxb. Ex. bausoned (with a white streak), baukie (a bat), blaud (to spoil), saut (salt), haud (hold).
4. It is joined with w to represent (1) [ɑ:] (see II. 1) and (2)  (See II. 3) generally in final position. Ex. blaw, caw, maw, raw (row of houses, etc.).
5. It is joined with e to indicate [e], [e1], [è] and [ɛ], long or short. Ex. tae (toe), maet (meet), blae, brae, haet (whit). The old sound of a, sometimes called the Continental a, was taught to the children in the dames' schools within living memory, and there is an echo of it in the name of the alphabet ah-bay-say in Orkney. See Dict. s.v. Note also the rhyme in Fergusson's Elegy on the Death of Mr David Gregory iv.: In Algebra weel skill'd he was, An' kent fu' well proportion's laws; He cou'd make clear baith B's and A's Wi' his lang head.