Reference page

Spelling: wa, qua

Pronounced with LOT vowel:

When a follows a w sound^[1] - spelled either w or qu (=kw) - then it is pronounced as o as in LOT:

• what, was, want, watch, wash, warranty
• quality, quantity
• squad, squat
• swan, swap, swamp, swat, swab, swallow, swaddle

Pronounced with NORTH vowel:

If there is an R in the spelling after a^[2], then it is pronounced as NORTH vowel:

• war, ward, warm, warm
• quarter, quartz
• swarm

As is the exception

• water

Pronounced with TRAP vowel:

If there is a velar sound after the vowel (i.e. k, g or ng)^[3], then it is pronounced as TRAP vowel:

• wax, wag, whack, wacky, wagon
• swag
• twang

As are the exceptions

• swam, wham, wigwam and twat

Why this pronunciation?

1. [w] rounds the lips forwards; if you try to say "wat" with an a, then repeat but hold your lips forwards throughout the vowel, you will hear it has moved closer to the rounded vowel [ɔ], o. Likewise, the back of the tongue is raised in [w], like in [ɔ] and unlike in [a]. Over time, this coarticulation became the standard pronunciation ↩

2. When Standard Southern British English lost its rhoticity - its pronunciation of final Rs - they were (eventually) converted into length. So words which had been pronounced with o+r became long o, as in NORTH ↩

3. I assume because it's harder to interpret the velar features in the vowel as deliberate when both the preceding and following consonants are velar. ↩