One of the mysteries of the English language finally explained.
An instrument for enlarging or reducing drawings; specifically one consisting of a ruler marked with two different scales placed end to end (one having markings more widely spaced than the other), which is pivoted about the point where the scales meet, so that measurements of a drawing can be taken using one scale, and consistently enlarged or reduced using the other. Now rare.
Late 19th century (in an earlier sense). From plani- + -graph.
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