American shoes are sized with a number and often a letter, and the letter describes width. What it actually measures is less obvious than the system's simplicity suggests.
Width is measured around, not across
The letter corresponds to the girth of the foot measured around the ball, at the widest point across the joints below the big and little toes.
A circumference rather than a flat width captures the volume of the foot, which is what determines whether a shoe fits rather than merely how broad the outline appears.
This is why two feet with identical footprints can need different widths, since one may be higher through the instep and displace more volume inside the shoe.
The letter shifts with length
Width designations are proportional. A given letter represents a larger girth on a longer shoe than on a shorter one, because feet scale in all dimensions together.
A size nine in a medium width and a size eleven in the same width therefore have different absolute girths, despite carrying the same letter.
Someone who changes length between brands can consequently find the same letter fitting differently, without either shoe being incorrectly made.
Men's and women's letters do not align
The standard width for men in the United States is designated differently from the standard for women, so the same letter denotes a different relative fit.
A women's shoe marked with the letter used for men's medium is a wide fitting, which is a persistent source of confusion when shopping across departments.
Doubled and tripled letters extend the scale outward in both directions, with each additional letter representing a defined increment in girth.
Most shoes are made in one width only
Every width requires a separate last, separate patterns and separate inventory, which multiplies cost for a manufacturer without expanding the total market proportionally.
Mass-market brands respond by producing a single medium width and accommodating variation through materials that stretch or through adjustable closures.
Athletic footwear does this most aggressively, using knit uppers that conform to a range of foot shapes rather than offering the range in the sizing.
Fitting changes through the day
Feet swell during the day as fluid accumulates, so a measurement taken in the morning can understate the volume a shoe needs to accommodate by evening.
Retailers that fit properly measure both feet, standing and weight-bearing, because the foot spreads under load and the two feet often differ.
Persistent pain, numbness or deformity is a matter for a podiatrist rather than a sizing adjustment, since fit problems and structural foot conditions produce similar complaints.