= Thread pitch
{title2=$p$}
\b[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.<Selecting the Optimal Screw Technology by Tolomatic>[Screw Terminology, printed pp5–6: pitch and lead]{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, so
$$
p=\frac{13.5\,\mathrm{mm}}{9}=1.5\,\mathrm{mm}.
$$
Dividing 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,
$$
p=\frac{1\,\mathrm{inch}}{q}
=\frac{25.4}{q}\,\mathrm{mm}.
$$
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 (hardware)> 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.
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