It is sometimes necessary to open or close the legs of the caliper to remove them from the object you are measuring. This can cause you to lose the measurement you have taken.

Transfer calipers have a secondary lock situated at the top of one of the primary legs.

The secondary leg is locked in place in line with one of the primary legs, before the caliper is removed from the object it is measuring.

The space between the secondary leg and the other primary leg is then measured with a ruler or micrometer.