While sizing may vary depending on the manufacturer, in most cases, a B in a woman's shoe size indicates a normal or regular-width shoe, and a B in a man's shoe is a narrow width. A normal or regular-width woman's shoe may also be indicated with an M.
For men, regular width shoes are usually sized with a D or a M, according to Boot Outfitters. Shoes can come in a variety of widths, including narrow to triple wide. The correct shoe width can be found by measuring the foot at the widest part, and it is best to get measured in a shoe store to find the proper fit.