A The inlet of the pelvis is widest in its antero-posterior diameter at about 11 cm.

B The mid-cavity is almost round, as the transverse and AP diameters are roughly similar.

C At the outlet, the AP diameter is about 12.5 cm.

D Station 0 means the head is at the level of the ischial spines, approximately mid-cavity.

E Station is documented as +/−1, 2, 3; +2 means the head is 2 cm above the spines.

F The coccyx may cause obstruction.