Twin boat, or Twin ship (Naut.), a vessel whose deck and upper works rest on two parallel hulls. -- Twin crystal. See Twin, n., 4. -- Twin flower (Bot.), a delicate evergreen plant (Linnæa borealis) of northern climates, which has pretty, fragrant, pendulous flowers borne in pairs on a slender stalk. -- Twin-screw steamer, a steam vessel propelled by two screws, one on either side of the plane of the keel.

* The relative position of the parts of a twin may be explained by supposing one part to be revolved 180° about a certain axis (called the twinning axis), this axis being normal to a plane (called the twinning plane) which is usually one of the fundamental planes of the crystal. This revolution brings the two parts into parallel position, or vice versa. A contact twin is one in which the parts are united by a plane surface, called the composition face, which is usually the same as the twinning plane. A penetration twin is one in which the parts interpenetrate each other, often very irregularly. Twins are also called, according to form, cruciform, geniculated, etc.

Still we moved
Together, twinned, as horse's ear and eye. Tennyson.

The life out of her body for to twin. Chaucer.