Thread pitch ($p$)

Thread pitch, usually $p$, is the axial distance between corresponding points on adjacent turns of a screw thread. It is measured parallel to the thread axis, rather than along the sloping helix. For a thread with several thread starts, this spacing differs from the screw lead, which measures advance per complete relative turn.^[ref]

The distinction between points and intervals is useful in a measurement. Suppose ten successive ridge crests along an axial line span $13.5\mathrm{mm}$ from the first crest to the tenth. They delimit nine pitch intervals, soDividing by ten would give $1.35\mathrm{mm}$ and would count the endpoints incorrectly. Measuring several intervals also distributes a fixed endpoint-position error over the number of intervals, provided the thread spacing is sufficiently uniform for that average to be meaningful.

A specification in threads per inch expresses the reciprocal spacing. For $q$ threads per inch,Thus $20$ threads per inch correspond to $p=1.27\mathrm{mm}$, using the exact inch-to-millimetre conversion.

For a two-start example with $p=1.5\mathrm{mm}$, the screw lead is $L=2p=3\mathrm{mm}$. A quarter-turn then advances a suitably constrained nut by $0.75\mathrm{mm}$ in the ideal geometry. A single-start thread with the same pitch advances only $0.375\mathrm{mm}$ in a quarter-turn. Equal pitch therefore does not establish equal motion per revolution.