shape coding
pp. Giving an object such as a control or a connector an appropriate or unique shape to make the object's function or usage obvious or to avoid confusion with similar objects that have different functions.
shape code v., n.

Example Citations:
Chapanis's design was revolutionary at the time, but it was subsequently generalized in the form of a design principle, known as shape coding, that can be used in all kinds of situations where it's important that controls be instantly identifiable.
—Kim Vicente, The Human Factor, Knopf, December, 2003

The ACD, however, requires minimal training to operate, with a much lower chance of an incorrect connection. While some training is required to ensure that the connection is made correctly, no special skills are necessary. The device design includes shape coding to ensure proper alignment and a blocking mechanism to prevent incorrect assembly.
—Holly Haughney, "A novel aseptic connection device," Pharmaceutical Technology, May 1, 2003

Earliest Citation:
To solve the problem with the airplane flap and landing gear switches, separate the switches and don't line them up in a row. Also use shape coding: a tire-shaped switch can control the landing gear, and the flap switch can be a long, thin rectangle—the shape of a flap.
—Donald O. Norman, The Psychology of Everyday Things, Basic Books, April, 1988

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