A shoe size is an indication of the fitting size of a shoe for a person. There are a number of different shoe-size systems used worldwide. While all shoe sizes use a number to indicate the length of the shoe, they differ in exactly what they measure, what unit of measurement they use, and where the size 0 (or 1) is positioned. Some systems also indicate the shoe width, sometimes also as a number, but in many cases by one or more letters. Some regions use different shoe-size systems for different types of shoes (e.g. men's, women's, children's, sport, and safety shoes). This article sets out several complexities in the definition of shoe sizes. In practice, shoes are often tried on for both size and fit before they are purchased. Deriving the shoe size Foot versus shoe and last The length of a person's foot is commonly defined as the distance between two parallel lines that are perpendicular to the foot and in contact with the most prominent toe and the most prominent part of the heel. Foot length is measured with the subject standing barefoot and the weight of the body equally distributed between both feet. The sizes of the left and right feet are often slightly different. In this case, both feet are measured, and purchasers of mass-produced shoes are advised to purchase a shoe size based on the larger foot, as most retailers do not sell pairs of shoes in non-matching sizes. Each size of shoe is considered suitable for a small interval of foot lengths, typically limited by half-point of the shoe size system. A shoe-size system can refer to three characteristic lengths: 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, 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 only gives the customer very