Small device I made to measure my feet when it came to ordering some custom Lake cycling shoes.
Simply slide the HEEL part until it stops on the start of the rule and the other end TOE just slide onto the rule and slide to your toe and check on ruler for measurement.
V's marks the point facing your heel and toe
Tip : if you dont have a standard 30cm ( 12") rule, try to get one where the sizes begin on the edge of the ruler.
The author hasn't provided the model origin yet.