A bay is a basic unit of library shelving. Bays are book cases about 3 feet (0.9 m) wide. Bays are stuck together in rows. Items are shelved from the top shelf to the bottom shelf in each bay.
Rows consist of a number of bays, either single-sided or double-sided, connected to each other. The standard length of a row is five to six bays, but it is not uncommon to find rows seven bays wide or even wider. In some countries a row is referred to as a 'stack' or a 'range'.