Coom or coomb or combe or coombe or comb?
Well, these are two or three different words, so you can have coom or coomb if you mean waste material from some vaguely industrial process, or you can have coomb, combe, coombe or comb if you mean one of the two different geological features those words cover.
Then there's the sloping ceiling you often find under a staircase or roof, which is spelled coomb.
The industrial waste sort of coomb will be, according to my Collins dictionary, something like coal dust or axle grease, and the geological feature depends upon whether you're in the south or north of Britain. In the south a combe will be a short valley or hollow, especially in chalky areas (places in Devon are quite often called Somethingcombe).
This is Ilfracombe: (you say it ILL-fr-COOM).
In the north a combe is one of these:
(this is White Coomb in the Highlands of Scotland)
that is, a semi-circular basin made by the erosion of a glacier - which actually I don't think you can really see in this picture. Rats.
Anyway, there we are. An easy spot, and a chance to choose your spelling.
Just what we need at the beginning of a working week.
Spot the Frippet: combe etc. The waste word started off meaning soot, and probably comes from culm, which is related to the word coal. The geographical word comes from the Old English cumb, and before that probably from a Celtic word (the Welsh cwm means valley).