How to use the thread pitch gauges ?

All the thread pitch gauges are used in the same way to identify internal or external threads. Each leaf has a number on it telling you how many threads per inch you have. The only exception to this is on the metric gauge. On the metric gauge, the number on each leaf represents the thread pitch (distance from the crest of one thread to the crest of the next thread.) in millimeters.
One important point illustrated above is the use of the correct thread pitch gauge on the threads. Incorrect thread identification can cause: equipment failure, leaking connections, thread damage to mating parts, or possible serious human injury.Additional thread identification is explained throughout this catalog.