Elongation in the Direction of the Clinodiagonal Axis
In cases where the basal pinacoid is fixed by some physical property like cleavage, the elongation is in the direction of the clinodiagonal axis. This crystal of Feldspar shown is a common example of this elongation.
George Huntington Williams, Ph.D. Elements of Crystallography for Students of Chemistry, Physics, and Mineralogy (New York, NY: Henry Holt and Company, 1892)