With n=1, the re-entrant walls are given of Borda's mouthpiece, and the coefficient of contraction becomes 2.
Two of these figures stood at the end of a re-entrant curve, several pieces of which are preserved.
This frequent twinning gives rise to characteristic forms, with many re-entrant angles, to which the names "spear pyrites" and "cockscomb pyrites" are applied.
3) has no re-entrant angles.
The herpolhode curve in the fixed plane is obviously confined between two concentric circles which it alternately touches; it is not in general a re-entrant curve.