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"Here one cube may be brought into the position of the other by a rotation of 180 degrees about a traid axis, or by reflection across the octrahedral plane which is perpendicular to this axis." -The Encyclopedia Britannica 1910

Interpenetrating Twinned Cubes

"Here one cube may be brought into the position of the other by a rotation of 180 degrees about a traid…

"...the two portions are symmetrical with respect to a plane parallel to the ortho-pinacoid." -The Encyclopedia Britannica 1910

Twinned Crystal of Gypsum

"...the two portions are symmetrical with respect to a plane parallel to the ortho-pinacoid." -The Encyclopedia…