Altern
AL'TERN adjective [Latin alternus, of alter, another.]
1. Acting by turns; one succeeding another; alternate, which is the word generally used.
2. In crystallography, exhibiting, on two parts, an upper and a lower part, faces which alternate among themselves, but which, when the two parts are compared, correspond with each other.
Altern-base, in trigonometry, is a term used in distinction from the true base. Thus in oblique triangles, the true base is the sum of the sides, and then the difference of the sides is the altern-base; or the true base is the difference of the sides, and then the sum of the sides is the altern-base.