Designating certain bonds and positions in a bipyramidal or pyramidal structure. For example, in a trigonal bipyramid the two bonds aligned through the central atom are the apical bonds and the groups attached to these bonds are in apical positions. The three other bonds and positions are equatorial. Similarly, in a square pyramidal structure the bond perpendicular to the base of the pyramid is the apical bond. The four other coplanar bonds (and positions) are said to be basal.