bone
To sight along an object or set of objects to check whether they are level or in line.
bone: sighting by eye to check alignment or level
To bone is to squint along the length or edge of a workpiece, using your eye as a crude sighting tool to detect deviation from flatness, straightness, or level. You align your line of sight with the object and look for gaps between the surface and your sight line, or look for wobble in what should be a straight edge. It requires no instruments, takes seconds, and costs nothing beyond the operator's attention.
The technique works because the human eye is surprisingly sensitive to discontinuities in a clean sight line. A carpenter bones a freshly planed board by sighting along its length from one end, low enough that the surface becomes a continuous plane against the background. A gap between board and sight line betrays high spots or wind. Similarly, boning across a carcass or along a frame edge reveals whether components are flush or proud.
Boning is a check, not a measurement. It tells you direction and rough magnitude of error, but not precise amounts. It is most useful for surfaces wider than 3 or 4 inches, where visual discontinuity is easy to spot. Narrow edges or thin shavings are harder to read by eye alone. The light matters too: side lighting, especially from a low angle, makes errors pop out; flat overhead light hides them.
The name likely derives from the straightness of bone as a reference object, or from the idea of laying a bone (or a stick, or a straightedge) alongside a surface to compare them. Historical use of actual bones as straightedges in woodworking is documented, though the verb almost certainly survived the practice itself. By the 20th century, boning was established slang among carpenters for this sighting method.
Boning sits between freehand judgment and instrument work. A winding stick or straightedge is more reliable over long distances or for critical joinery. A level or laser is needed for absolute measurement. But boning remains the fastest, most portable, and least dependent check in the shop, and experienced carpenters trust their eye over their tools for many everyday decisions about fit and finish.