What is the best instrument to measure the inside width and depth of a keyseat?

The best instrument to measure the inside width and depth of a keyseat is a dial caliper. Dial calipers are specifically designed for precise measurements and can handle a variety of dimensions, including depth and internal dimensions, thanks to their contrasting jaws and depth probe. The dial readout also provides a clear visual reference, making it easier to ensure accurate measurements.

Dial calipers can measure not only the width of the keyseat but also its depth, which is crucial in applications where precision is essential. They typically offer a measurement range suitable for most keyseat sizes and can be used with a high degree of accuracy.

In contrast, while a calibrated ruler can provide basic measurements, it lacks the precision and versatility needed for such specific tasks. A micrometer is excellent for measuring small dimensions, particularly external ones, but is not as suitable for measuring the internal dimensions of a keyseat. A vernier scale can also measure with some accuracy, but it doesn't offer the ease of reading and versatility in measuring depths compared to a dial caliper.