[8] Other common terms used in machining with Imperial units involve adding tenths together to achieve a specific tolerance or measurement.

1 thou is equal to: The introduction of the thousandth of an inch as a base unit in engineering and machining is generally attributed to Joseph Whitworth[13] who wrote in 1857: ... instead of our engineers and machinists thinking in eighths, sixteenths and thirty-seconds of an inch, it is desirable that they should think and speak in tenths, hundredths, and thousandths ...[14]Whitworth's main point was to advocate decimalization in place of fractions based on successive halving; but in mentioning thousandths, he was also broaching the idea of a finer division than had been used previously.

Until then, workers such as millwrights, boilermakers, and machinists in the Anglosphere measured only in traditional fractions of an inch, divided via successive halving, usually only as far as 64ths (1, 1⁄2, 1⁄4, 1⁄8, 1⁄16, 1⁄32, 1⁄64).

Dimensions and geometry could be controlled to high accuracy, but this was done by comparative methods: comparison against templates or other gauges, feeling the degree of drag of calipers, or simply repeatably cutting, relying on the positioning consistency of jigs, fixtures, and machine slides.

During the following half century, such measuring instruments, previously expensive rarities, became widespread, everyday tools among machinists.