You can pick up nearly any mineralogy book, open it to the discussion of "parting," and you will usually find something like the following: "Twin crystals, especially polysynthetic twins, may separate easily along the composition planes" (Hurlbut 1949), or "Many substances tend to break readily along the twin plane" (Berry and Mason 1959), or "A cleavage-like fracture due to separation between twins is parting" (Sinkankas 1964), or "These are likely to be planes of weakness in the ...