Generally oriented mesas and pyramidal-shaped objects were patterned in (1 0 0) InP substrate by etching in 3HCl:1H3PO4 at (16 ± 0.05) °C through convex InGaAs mask patterns. The mesas were revealed through long (∼2 mm) parallelogram-shaped mask strips whose long edges lay between [0 1 1] and at a variable angle ω, with ω ≡ 0° at [0 1 1]. The mesas were re-entrant for ω ϵ [0°, 45°) and ordinary for ω ϵ (45°, 90°]. The tilt of mesa side facets α and the associated mask underetching rates were evaluated using SEM, AFM and optical microscopy. The underetching rate was zero at ω = 0° and 90°. With |(ω − 45°)| → 0° it increased asymmetrically about [0 0 1]: for |(ω1 − 45°)| = |(ω2 − 45°)| it was higher at ω1 < 45° than at ω2 > 45°. At ω = 45.5° ordinary (1 1 0)- and -related facets, tilted at α ≈ 45°, were revealed at a rate of (310.1 ± 8.1) nm min−1. Likewise at ω = 45.5° off [0 1 1] to (1 0 1)- and -related facets formed. In theory, (1 1 0), , (1 0 1) and define a pyramidal shape with the sides tilted at 45° to (1 0 0). In practice, an object confined by facets related to (1 1 0), , (1 0 1) and cannot be revealed via a convex-cornered lozenge-shaped pattern with the edges at ω = 45.5° and diagonals parallel with [0 1 1] and because the ‘pyramidal’ facets are suppressed by fast-etching re-entrant facets that form at the - and -oriented corners. If compensated with e.g. rectangular strips, the pyramidal facets can be revealed at the lozenge edges. Etch-stop (2 1 1)A-related facets were formed along the compensation strip edges and fast-etching re-entrant facets at the convex corners of the compensation strips. The angular orientation of the pyramidal facets slightly shifted with etching time: the normal vector of the top pyramidal facet edge deviated off ⟨0 0 1⟩ towards [0 1 1] or from (1.13 ± 0.14)° at tetch = 2 min to (2.32 ± 0.10)° at tetch = 10 min. Concurrently, the tilt of the ‘pyramidal’ facets α to (1 0 0) decreased from (45.35 ± 0.04)° for tetch = 2 min to (44.16 ± 0.06)° for tetch = 10 min.