Veritas Carpenter's Gauge
Specifically designed to be carried in an apron or pocket, this is both a direct-reading compass and a marking gauge, two things that carpenters frequently need but find hard to carry.
It has a rotatable head, which locks vertically for use and horizontally for storage and tip protection. It is direct reading, for both radius and diameter, in inches and centimetres. If you have ever measured a dryer vent pipe at 5-7/16 In. diameter and needed to cut a hole for it, you'll know why direct-setting diameters are such an advantage. Similarly, if you have ever had to put a radius on a corner, you know why a combination compass/marking gauge is so valuable. You set your radius, locate the center using the gauge, then rotate the head and scribe your line with the compass.
The gauge is also useful in a cabinet shop because of the direct-reading feature (substantially reduces errors), as well as for the fast corner radiusing just described. Made of brass, stainless steel and high-impact ABS, it is 7-1/2 In. long overall, and will scribe circles up to 12 In. diameter.