(Update: I've just updated this post for two reasons: a) The Blixy Zoom Words isn't working so I've used plain old writing; b) I needed to make it clear this post was about British English pronunciation)
Look at these words. What do you notice about the sound and the spelling?
wander, want, watch, wash, swallow, warrior, swap, wannna
And now these:
walk, wall, war, warm, water, wardrobe, warning, towards, dwarf
Hopefully you noticed that all the words begin with, or contain, the letters wa.
In British English in the first lot, the wa sounds like it should be spelled wo - we hear the sounds /wɒ/ : watch, swallow, was, etc.
In the second graphic, the wa sounds more like 'wor' (war) or /wɔ:/ walk, warning, award. But if you look carefully, you'll see that the wa in these words is mostly followed by r or l (except in water).
Are these rules? No, they can't be rules because they are not definite enough. There are exceptions such as water and also a very few words where the wa makes a more phonetically regular sound of /wæ/ like in wax (and another word that's not polite enough for The Spelling Blog!) and a few other pronunciations. But there are patterns to be seen.
Are these patterns useful? Yes, I think it's worth raising awareness of this. Nobody ever pointed this out to me and I just thought a lot of these words were phoneticaly irregular, but it's reassuring to see a pattern.
However I would feel happier if I could find an etymological reason for this pattern - believe me, I've searched. Any ideas, anyone?
By the way, when I finally finish my Teaching English Spelling book (before summer 2010, I hope), there will be an awareness-raising exercise about the wa spellings.