A hairpin must vertically align with, and horizontally abut, the dynamic marks that bracket it

A crescendo/decrescendo wedge is drawn at the same baseline height as the dynamic letters it connects, and its narrow point should nearly touch the adjacent dynamic's glyph edge.

A hairpin from p to f is drawn at a slightly different vertical height than the 'p' and 'f' letters flanking it.

Ross, The Art of Music Engraving, dynamics/hairpin alignment section; Gould, Behind Bars, ch. 'Dynamics' — hairpins share the dynamics baseline.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.