This tempered cast iron square head fits 12" to 24-inch combination square blades SPI accessories for combination squares expand the function of your existing measurement tools This is a replacement or add-on accessory: the combination square blade is not included Interchangeable with all popular national brands of combination squares SPI Square head for combination squares. Interchangeable with all popular national brands of combination squares. Fully hardened and tempered for long lasting durability and reliability. Fits combination squares with 12 to 24 inch blades. Square head only (blade and other head styles are not included).