SPI machinist square made from hardened spring steel and manufactured to exacting specification 12" Blade Length, 7-1/2" Base Length, 0.001" Parallel Accuracy; 0.002" Square Accuracy Precision ground to ensure straightness and parallelism; working edges of each blade are lapped for accuracy Hardened spring steel blade is permanently secured to the base to ensure rigidity SPI 13-629-1 Machinist Square: Hardened Spring Steel. 12" Blade Length, 7-1/2" Base Length, 0.001" Parallel Accuracy; 0.002" Square Accuracy. Working edges of each blade are lapped for accuracy.