Hand-tool lesson · Measure and mark

Combination square

Use a reference edge to mark a square line across a board.

Watch · practice · showInstructor-supervised
All hand-tool lessons
In this lesson

Know the tool

Maplewoodshop photograph of a combination square with a metal rule and sliding head.

Your goal: Keep the square's head seated while drawing and checking a 90-degree layout line.

Find the rule, sliding head, and locking knob. The head's square reference face sits against the board's edge; the rule crosses the face of the board.

Identify the 90-degree reference face. Some heads also have a 45-degree face; the two faces serve different layouts.

Watch and prepare

At your station

  • A combination square, pencil, tape or ruler, and a flat supported scrap with a sound reference edge.
  • An instructor-checked square for checking the result.

Demonstration and setup

  • Watch the instructor seat the correct head face against the edge, with the rule resting across the board.
  • Loosen the knob only enough to adjust the rule, then lock it before marking.
  • Check the edge first. A gap or rough reference edge can make a misleading line even when the tool is accurate.

Practice

Start after the demonstration and the instructor's setup check. Stop and ask for help if the work moves or the tool does not behave as demonstrated.

  1. Mark a position.

    Measure 3 inches from the chosen end and make a small V. This is a layout exercise; keep the saw put away.

  2. Seat the head.

    Hold the reference face firmly against the sound edge as demonstrated. Move the square until the rule passes through the V point.

  3. Draw the line.

    Keep the head seated and draw a narrow line along the rule. Keep fingers clear of the pencil's path.

  4. Check the angle.

    Use the checked square to inspect the line from the same reference edge. If it disagrees, check the seating, edge, and tool with the instructor.

Show your skill

Show the instructor these observable results before using the skill in a project:

  • The head stays against the reference edge, rather than resting loosely on top.
  • The line passes through the V and is square to that edge.
  • You can explain how a gap under the head changes the result.

Check your understanding

What angle does a square crosscut line make with its straight reference edge?

Reveal the explanation

90 degrees, a right angle. This describes the layout line; sawing accurately along the intended side is a separate skill.

Answer aloud or on paper, then compare with the explanation.

Use the skill

Apply it in a project

Maplewoodshop materials adapted with permission. Images © Maplewoodshop. Practice tasks and learning checks by STEM Guides. Maplewoodshop.