(A quick refresher here on screw sizes: The first number is the size of the fastener, such as Number 10, 8, or 6; the second number is the threads per inch, such as 24, 32, or 40.) ...

Thus, a tap or die marked 1/4-20 NC cuts threads for a 1/4-inch-diameter screw or bolt with 20 coarse threads per inch. Taps and dies with diameters less than 1/4-inch are numbered according to ...