There are two methods for measuring changes in penile rigidity and circumference during nocturnal erection: snap gauge and strain gauge.

The snap gauge has four anvils or jaws, the first one or pair (outermost) are set using the upper limit (tolerance) of the part and the inner set adjusted to the lower limit of the part.

Snap gages have been in existence and referred to by that name since at least 1898.

In a manufacturing plant, snap gages would normally be re-calibrated at scheduled intervals.

Snap gages facilitate making interchangeable parts.

Some vendors who resell a different measuring instrument, bore gauge, muddy the waters by referring to them as snap gages, unlike the manufacturer who uses the correct nomenclature.

Often, snap gages will be calibrated by skilled workers and used by semi-skilled workers; the snap gage is faster than a micrometer and requires less skill to use correctly.

Adjustable snap gages may be reset to compensate for wear or re-tasked for measuring a different dimension and can be purchased off the shelf and set rather than needing to be fabricated from scratch when a new gage is needed.

Snap gages may be machined and ground out of a single block of metal or adjustable snap gages, which have movable anvils that may be adjusted over a limited range of sizes, may be used.