[f. TWIN v.2 + -ING1.] The action of TWIN v.2
1. Production of two children or young at a birth; bearing of twins.
1573. Tusser, Husb. (1878), 81. Ewes yeerly by twinning rich maisters doo make.
1822–9. Good, Study Med. (ed. 3), V. 226. In Congruous Twinning, or ordinary twin cases, in which there is no disparity of size between the two.
1883. Duncan, in Brit. Med. Jrnl., I. 497. In the mare, twinning is a far rarer event than in woman and the cow.
2. Coupling, close union or combination; spec. in Cryst. the union of two crystals so as to form a twin crystal (see TWIN sb. 3 b).
1845. [see TWIN sb. 3 b].
1879. Rutley, Study Rocks, x. 87. This twinning is frequently … many times repeated in the felspars.
1898. Naturalist, 176. A zonal structure as well as twinning—both on the pericline and albite plans.
3. attrib., as twinning-axis, -law, plane, Cryst. = twin-axis, -law, -plane (see TWIN C.); twinning-machine, -saw, names of apparatus for cutting two combs from a single piece of material.
1875. Knight, Dict. Mech., Twinning-machine, a machine for cutting two combs (twins) from the single piece…. Twinning-saw.
1883. Science, I. 331/2. The twinning plane is parallel to the ortho-pinacoid.
