There are three characteristic lengths that a shoe-size system can refer to:
The median length of feet for which a shoe is suitable. For customers, this measure has the advantage of being directly related to their body measures. It applies equally to any type, form, or material of shoe. However, this measure is less popular with manufacturers,[citation needed] because it requires them to test carefully for each new shoe model, for which range of foot sizes it is recommendable. It puts on the manufacturer the burden of ensuring that the shoe will fit a foot of a given length.
The length of the inner cavity of the shoe. This measure has the advantage that it can be measured easily on the finished product. However, it will vary with manufacturing tolerances and provides the customer only very crude information about the range of foot sizes for which the shoe is suitable.
The length of the "last," the foot-shaped template over which the shoe is manufactured. This measure is the easiest one for the manufacturer to use, because it identifies only the tool used to produce the shoe. It makes no promise about manufacturing tolerances or for what size of foot the shoe is actually suitable. It leaves all responsibility and risk of choosing the correct size with the customer. Further, the last can be measured in several different ways resulting in different measurements.[1]
All these measures differ substantially from one another for the same shoe.
Length unit
Sizing systems also differ in what units of measurement they use. This also results in different increments between shoe sizes because usually, only "full" or "half" sizes are made.
The following length units are commonly used today to define shoe-size systems:
The Paris point equals to ⅔ centimetres (6.6 mm or ~0.26 in). Usually, only full sizes are made, resulting in an increment of ⅔ centimetre. This unit is commonly used in Continental Europe.
The barleycorn is an old English unit that equals to ⅓ inch (8.46 mm). Half sizes are commonly made, resulting in an increment of 1⁄6 inch (4.23 mm). This unit is the base for the English and the U.S. sizing system.
Further, metric measurements in centimetres (cm) or millimetres (mm) are used. The increment is usually 0.5 cm (5 mm or ~0.20 in), which is between the step size of the Parisian and the English system. It is used with the international Mondopoint system and with the Asian system.
Due to the different units of measurements, converting between different sizing systems results in round-off errors as well as unusual sizes such as "10⅔".
