Coarse and Fine
There are two grades of threads per inch: coarse and fine. For each specific bolt size, there are two possible thread types. One bolt --- or machine screw --- has more threads per inch, which is the fine option, and the other has fewer threads per inch, which is the coarse option.
UN Thread Types
Although metric, British and American systems for describing threading are different, there is a UN --- or unified --- system that uses a threads per inch count. There are variations on the UN measurements. For example, UNJ threads are slightly larger than the UN diameter. The UNF is a finer thread, with more threads per inch, and the UNEF is extra fine thread, with even higher counts per inch. For someone visiting a home repair or automotive repair store, UN threading is the standard.
Some common sizes of bolts and threads include #00 and #0, which come in fine thread only, 90 and 80 threads per inch respectively. Size #1 threads per inch is 64 in coarse and 72 in fine, while size #3 is 48 in coarse and 56 in fine. For #5 fasteners, threads per inch are 40 in coarse and 44 in fine, while #8 is 32 in coarse and 36 in fine. A 1/4-inch fastener has 20 threads per in in coarse and 28 in fine, while a 3/8 fastener has 16 in coarse and 24 in fine.