According to Joe AutoDrill :
Okay... I understand why fractional drill sizes and metric drill sizes
exist... They are common sense to me.

But... Why do number and letter drill sizes exist? Do they correspond to a
wire size or some other standard?

Mostly, they fill in the gaps in the fractional sizes, and came
into being before the Metric drills became easier to get here.

The number sizes are at least related to wire sizes -- the more
times through the drawing dies, the smaller the physical size. Letter
size seem to go the opposite direction, but again fill in gaps in the
fractional sizes, so it is easier to make a hole the size you actually
*need* (from strength details, rather than just to match whatever screw
you have handy -- or to make a hole which allows a specified amount of
liquid or gas to pass per unit time).

Of course, the number sizes and letter sizes often come closer
to being the optimum size for a tap drill for common Imperial screw
sizes. Metric sizes tend to work out pretty well with just the metric
set in 0.1mm steps.

Enjoy,
DoN.

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