Top, Meikle Mine, Nevada, USA; left, Leiping Mine, Hunan, China; right, Wuzhou Mine, Guangxi, China; bottom, Torr Works Quarry, Somerset, UK.

Class 1 with c-axes parallel (twin plane {00.1}): Meikle twin (top) in the photo. Common according to Dana and I agree; I've seen lots of these, in both contact and penetration twin forms.

Class 2, c-axes are inclined at a little over 127 degrees (twin plane {01.8}): Wuzhou twin (right) in the photo. Very common according to Dana, but I've not seen that many of these (though they are easily formed with some pressure from a knife as shown in the Dana drawing).

Class 3, c-axes are inclined at a little less than 91 degrees (twin plane {10.4}): Leiping twin (left) in the photo. Not common according to Dana, but I've seen many examples of these.

Class 4, c-axes are inclined at a little less than 54 degrees (twin plane {01.2}): Torr works twin in the photo (bottom). Rare according to Dana and I agree.