Combination square: versatile measuring and marking tool with multiple functions. Components: STEEL RULE (typically 12 inches with inch markings); STANDARD HEAD with 90 degree edge and 45 degree edge that clamps to rule at any position; SOMETIMES includes built-in level vial; SOMETIMES includes a scribe (small pointed marker); optional accessory: center finder head (for finding center of round stock), protractor head (any angle). Functions: mark 90 degree (perpendicular) lines for cross-cuts by placing head against edge and marking along rule; mark 45 degree lines for miter cuts (45 degree face on head works similarly); measure depths (extend rule into recess, head sits on surface); measure heights or clearances; set depth of saw blades or router bits; use level vial to check level/plumb; find center of round stock with center finder; check inside corners are square; establish lines parallel to an edge at consistent distance.
Proper use: ensure rule is clean and head clamps tight; reference edges of head against work edge; mark with sharp pencil or knife along rule edge. Quality: a good steel combination square (Starrett brand is standard) is reasonably priced and lasts for life. Test a new square: mark a perpendicular line against a known-straight edge, then flip the square over and mark another line from same point; if they don't overlap, the square is off.
Other measuring/layout tools: SPEED SQUARE (Swanson) is triangular tool to quickly mark 90 and 45 degree lines and lay out rafter angles using roof pitch scale; common in framing. FRAMING SQUARE is L-shaped large square (16 inch by 24 inch) used to lay out stairs, rafter cuts, large layouts; tongue and blade; tables stamped on it for stair stringers and rafter cuts. MITER GAUGE slides in table-saw slot and holds workpiece at angles for crosscuts. MITER BOX has slot guides for hand-sawing accurate angles. MARKING GAUGE has single pin that scribes lines parallel to edge at set distance; common in joinery. PROTRACTOR measures any angle in degrees. BEVEL GAUGE or SLIDING T-BEVEL has sliding blade that locks at any angle; transfer angles from one place to another. SQUARE-AND-COMPASS for geometric layout. DIVIDERS measure or transfer equal distances.
Layout for carpentry: establish reference faces/edges (marked, used as reference for measurements); don't measure from edges that have been cut from rough lumber — use jointed/cut reference faces; mark all cuts before cutting and verify; for repeated cuts of same length use a stop block on saw fence; for 90 degree crosscuts in repetitive work use miter saw (chop saw) with fence; dog-ear symbols (cross marks) indicate the waste side of a cut line.
Plumb and level: plumb is vertical (perpendicular to gravity); level is horizontal. Tools: level (bubble vial; sometimes called spirit level from old times when filled with alcohol); plumb bob (weighted line for vertical); laser level (modern; projects level or plumb lines). Digital levels show angles numerically.