They have two primary purposes, comparing measurements (such as ensuring equal diagonals in drawers or carcasses to guarantee squareness) and transferring measurements.
Any time you can transfer a dimension directly, without converting it into numbers, you avoid a primary source of error.
Bar gauges not only shine in basic measurement transfer, but are unequalled for inside measurements, such as for sizing shelves or interior trim in windows. They can also be used as "story sticks" for complex built-ins where nothing is square.
Gauge head openings are 1/2" x 3/4"; you just rip 1/4" slices off standard 3/4" lumber to make bars of the length you want (wood is not included). Made of brass and ABS plastic. We now include two brass pins for transferring inside or outside measurements smaller than the length of the sticks or for transferring outside measurements without making hook tips. A pair of brass pins is available separately to retrofit a previously purchased set of bar gauge heads (although a pair of small nails will also do the trick).
Made in Canada.