The throat depth (also known as the effective depth or waist) is the perpendicular thickness of uncut lumber remaining between the bottom edge of the stringer board and the inside notch intersection.
The 3.5-Inch Code Mandate
Most building departments require a minimum throat depth of at least 3.5 inches (89 mm) of solid, uncut wood. If you cut standard 7.5-inch risers and 10.5-inch treads into a 2×10 (which has an actual width of only 9.25 inches), your remaining throat will often drop to 3.0 inches or less, triggering an automatic inspection failure.