s. When more than three strands are required in a
compound thread it is customary to pass the material more than once
through the doubler, as, for example, in a sixfold thread, two strands
may be first twisted together in the same or in the opposite direction
to the spinning twist; after which the once-doubled thread is
"cleared," folded, and three strands of twofold yarn are twisted in
the opposite direction to that employed in the first operation. In
some machines folding and twisting proceed simultaneously, and some
are furnished with an automatic stop motion. But when twisting two
threads together to oppose the spinning twist, the failure of one
causes the other to untwist and break, therefore, under such
circumstances a stop motion is unnecessary.

Intermittent doublers are known as twinners, and these are of two
kinds, namely, English and French. In the former the spindles are
fitted in a stationary rail, but the creel, containing the cops or
ring spools, is mounted upon a carriage and moves in and out, as in
Hargreaves' spinning jenny (see SPINNING). French twinners have a
stationary creel, and the spindles move in and out with the carriage,
as in the spinning mule. The material to be folded is often subjected
to the action of steam in order to render it less resilient, after
which it is mounted upon skewers in the creel, and two or three
threads are passed to each spindle to be twisted together and formed
into a cop. Between the creel and the spindles all the strands are
kept equally tense by drawing them over flannel-covered boards and
under porcelain weights. For wet doubling, the strands pass through a
trough containing water, and the flannel surfaces are also wet.

_Clearing_.--After the first, or the final, doubling it is often
necessary to remove lumps, imperfect knots and loose fibres from a
thread. This is accomplished by passing each through a slit, or
clearer, whose width is adjusted to the diameter of the thread to be
treated. By this means anything which gives a thread abnormal bulk
will be prevented from passing the slit. Once through the slit, a
thread is coiled upon a friction-driven, double or single-headed
bobbin. If the former, the coils are evenly laid; if the latter, they
are disposed into a bottle shape. Or, again, cheeses may be wound.

_Gassing_.--In cases where a thread with a smooth surface is required
the extendin