"There are... fixed boundaries, beyond and about which that which is right cannot exist."
Horace, Satires, Book 1 (35 B.C.E.)
Vernier callipers are a sliding, adjustable-jaw device for measuring distances of a few centimeters to an accuracy of 0.01 centimeters. A main (ruler) scale is marked off with 0.1 centimeter divisions. Sliding parallel and alongside the main scale is a much smaller vernier scale on which ten divisions are equally spaced over 0.9 centimeters of the main scale. To subdivide the 0.1 centimeter division on the main scale into ten, the user has to select the nearest vernier division that is in line with one of the main scale divisions.
The scale was invented by French scientist and engineer Pierre Vernier (1580-1637), and the details were published in his 1631 book, La Construction, I'usage, et les proprietes du quadrant nouveau de mathematiques (The Construction, Uses, and Properties of a New Mathematical Quadrant), published in Brussels. Vernier was interested in cartography and surveying, and his vernier was first used on the circular scale of a quadrant theodolite. The scale enabled angles to be measured with ease to an accuracy of one minute of arc (one-sixtieth of a degree). Unfortunately, dividing a circular scale into an equal number of uniform degrees was a matter of considerable complexity and vernier scales did not become a common adjunct to angle-measuring devices such as telescopes and theodolites until the early nineteenth century.
