A really daft question here but can anyone explain what Drill sizes A-Z are used for, you expect to see this sort of thing around the start of April but every day is a school day...
They're unusual looking sizes that match threading taps, wire gauges and the like, there are also number drills with similar uses - the letter and number sequences are to save marking them with long decimal sizes Vs fractional inches or mm.
Letter and number drills are based on SWG (standard wire gauge), they where once very common especially for tapping and clearance drills for odd ball threads like BA. Also they where used for solid rivets as rivets diameters where also based on SWG.
Nowadays they are almost defunkt especially as metric bits are easily avalable in 0.1 increments up to 13mm
I got hold of a set of both number and letter drills by asking the bloke who had bought them in a lot at auction, if he had bought the lot for them or the books,he didn't even know they were there and he accepted my offer of a fiver