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Multiple heddles

Rigid heddles (above) are called "rigid" to distinguish them from string and metal heddles, where each warp thread has its own heddle, which has an eye at each end and one in the middle for the warp thread. The eyes in the ends are fastened to a shaft, all in a row. This requires multiple shafts; it cannot be done on a single-shaft loom. The different shafts (also called harnesses) must be controlled by some mechanism.

While non-rigid heddles generally mean that two shafts are needed even for a plain tabby weave, twill weaves require three or more (depending on the type of twill), and more complex figured weaves require still more harnesses.