An X-shaped divider, formed by joining a pair of two-pointed arms which is used for enlargement and reduction.

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A proportional divider lets you scale drawings up or down while keeping the same proportions. Its X-shaped form comes from two arms that each end in two points and are joined at a hinge. By opening them to set a fixed ratio, one pair of points can transfer a dimension from the original size to a new size with the same proportion. So, when you place the tips on a measurement on the source, the other tips give the corresponding enlarged or reduced measurement on the target. This is exactly what’s needed for enlargement and reduction. Ruler measures straight lengths, caliper measures with sliding jaws, and a compass draws circles—none of them enforce a proportional change of size across a drawing.