The Brannock device measures feet to determine correct shoe size. Separate devices exist for kids and adults, as well as for men and women, and a combination device is available for both genders as well.
To use the tool, push the width bar to its widest position, and slide back the arch indicator. Place your right foot with socks on in the device with the heel in the right heel cap. Take the length of the longest toe to measure the heel to toe length. Hold the ball joint of the foot with the thumb and push the pointer ahead, so the pointer's inner curve fits the foot's ball joint, and the two high ribs touch the thumb. The lower middle rib on the side of the foot measures the arch length.
The greater length of the heel-to-toe length and the arch length indicates the correct shoe size. To measure the width, push the width bar to the side of the foot, and locate the shoe size on the movable width bar. Read the width measurement indicated. Repeat the process with the left foot, and compare the two shoe sizes. Take the bigger size to be the correct shoe size.