T-slots are provided on most mill tables for fixturing work and workholding devices. These slots are designed to work with T-nuts and are made to either ANSI or ISO standards. See ANSI B5.1M or ISO 299. T-slots can be made to tolerances suitable for holding only to tighter tolerances intended for locating.
T-nuts are designed to work inside of T-slots. ANSI/ASME B5.1M defines T-slot size limits. See Also ISO R286.
Rotary T-nuts are available that can be inserted from the top of the table instead of at the end.