In a unit cell of a crystal lattice the angle beta is between faces:

Explanation

By convention the three interfacial angles of a unit cell are alpha between the b and c axes, beta between c and a, and gamma between a and b, each angle being named after the axis it does not lie against. Together with the three edge lengths, these six parameters define the cell completely. The Greek letter did not print cleanly in the original paper, appearing there as a stray character.