1-2-3 Blocks

A 1-2-3 block is a rectangular precision reference block machined to exactly 1" x 2" x 3", with every face flat, square to its neighbors, and parallel to its opposite face, typically to a few ten-thousandths of an inch. They originated in machinist work, where that level of precision is routine, but they've become a genuinely handy shop accessory well beyond machining — a known-good, known-flat, known-square reference object is useful anywhere you need to check or set up against a trustworthy standard.

What they're actually used for in a woodshop

• Setup blocks — stacking or combining known dimensions (1", 2", 3", and combinations of those) to set fence distances, blade heights, or router bit depths without measuring off a potentially inaccurate ruler mark.
• Checking square — because the faces are precision-ground square to each other, a 1-2-3 block is a reliable reference for checking whether a jig, fence, or assembly is actually square, not just close.
• Clamping and support — a rigid, perfectly flat spacer when gluing up or supporting a workpiece at a known, consistent height.

Why not just use a tape measure

A tape measure and a machinist's precision block are solving different problems. For most cuts, a tape measure is entirely adequate. For setup steps where being off by even a few thousandths compounds across a project — repeated identical joints, stacked dado depths, aligned shelf-pin holes — a physical reference you can butt a fence or bit against removes the cumulative error a tape measure's hook slop and reading interpretation can introduce.

Buying notes

Most 1-2-3 blocks are sold in pairs, since a huge part of their usefulness comes from stacking or spanning between two identical blocks. Ground steel is the standard material; some are drilled and tapped with holes for fixturing, which matters more in machining than in a typical woodshop.