 BB84 uses two pairs of states, with each pair conjugate to the other pair, and the two states within a pair orthogonal to each other.Pairs of orthogonal states are referred to as a basis.The usual polarization state pairs used are either the rectilinear basis of vertical (0°) and horizontal (90°), the diagonal basis of 45° and 135° or the circular basis of left- and right-handedness.