Pitch is the number that decides what nut fits: an M10×1.5 thread has a basic major diameter of 10 mm and a pitch of 1.5 mm. ISO 261 lays out the general plan of metric pitches and ISO 262 selects the ones fasteners are actually made in — every size M3–M30 has its coarse pitch, the fine selection only starts at M8, and the selection is deliberately narrower than the full ISO 261 series, which offers extra-fine options like M6×0.75 that standard bolts are simply not manufactured in. Unified inch threads carry the same information inverted, as threads per inch, so pitch there is 1/TPI.
At the drill press, pitch sets the tapping hole. The Machinery’s Handbook percentage-of-thread formula is drill = major − 1.29904 × pitch × (%/100), and the classic metric shop rule — tap drill equals major minus pitch, 8.5 mm for M10×1.5 — is that same formula evaluated at exactly 76.98% engagement, slightly tighter than the 75% the published charts standardize on but well inside the safe range.
In strength calculations pitch appears as a penalty: threads remove material, so a bolt’s tensile stress area is At = (π/4)(d − 0.9382·p)², a diameter between the pitch and minor diameters rather than the nominal shank. For an M16 with its 2.0 mm coarse pitch that comes to 156.7 mm² — the area every proof-load and preload figure for that bolt is built on.