How is wooden furniture made?

1. Every piece of furniture starts as a drawing. The plan shows each part and exactly how long, wide and thick it has to be.
2. Next, each piece is measured and marked on the board with a sharp pencil. Woodworkers say: measure twice, cut once. You can cut a board shorter, but never longer!
3. The saw follows the pencil lines. Soon all the parts are ready: four legs the same length, the rails that join them, and a seat.
4. The pieces lock together with joints. Here a tongue on the end of the rail, called a tenon, slides into a pocket in the leg, called a mortise. Glue goes in first.
5. Now everything is fitted together. Clamps squeeze the joints tight while the glue sets. After a few hours the pieces act like one solid thing.
6. Last, the stool is sanded smooth and given a finish, such as oil, wax, varnish or paint. The finish makes the wood look rich and keeps out water and dirt.

Tap each part of the stool and the close-up of its joint. Watch how they fit.

• Seat. The seat is a flat board that spreads your weight out to the four legs. Its edges are rounded so they feel comfortable.
• Legs. The four legs carry your weight down to the floor. They must be exactly the same length, or the stool will rock.
• Rails. Rails join each leg to the next one. They hold the legs square so the stool does not sway from side to side.
• Mortise and tenon. The end of the rail is cut into a tongue, the tenon. It fits a pocket in the leg, the mortise. Because the pieces lock into each other, the joint is very strong.
• Dowels. Dowels are short round pegs. Half of each peg is glued into one piece and half into the other, holding the seat on with no nails showing.
• Finish. The finish is a thin protective coat. It keeps water, dirt and sticky fingers out of the wood and makes the grain look beautiful.

Choose a joint and load it with weights until it breaks or holds them all. Test at least two joints. Which is strongest?

Experiment: Which joint will hold the most weight?

1. Start with the butt joint. Press Load it up and count the weights it holds.
2. Now choose the dowel joint, then the mortise and tenon, and test each one.
3. Press Record after each test and compare.

What we found: The butt joint snaps off first, because only a thin line of glue on the end of the rail holds it. Dowels help. The mortise and tenon is strongest: the tongue is locked inside the leg and has lots of room for glue.

Turn on JavaScript to try it yourself.

Build a kitchen stool from the plan. Cut the leg and the rail to the right lengths, pick a joint and a finish, then press Build and test.

A wooden stool looks like one solid thing, but it is made from many separate pieces: legs, rails and a seat. The furniture maker's job is to cut each piece to exactly the right size and then join them so well that they act like one.

It starts with a plan. The plan lists every piece and its measurements. Then each piece is measured, marked with a pencil and cut. Woodworkers have a saying: measure twice, cut once. A piece that is too long can be trimmed, but a piece that is too short is wasted. And if one leg is shorter than the others, the stool wobbles.

Next come the joints. You could just glue the flat end of a rail against a leg, but that joint, called a butt joint, snaps off easily. A much better way is to cut a tongue on the rail and a matching pocket in the leg. The tongue is called a tenon and the pocket is called a mortise. Now the wood itself holds the pieces together, and the glue has much more surface to grip. Clamps squeeze everything tight until the glue is hard.

Last comes the finish. Bare wood soaks up water, juice and dirt, and it swells when it gets damp. So the maker sands the stool smooth and adds a coat of oil, wax, varnish or paint. Our toy brick comparison is not perfect: bricks pull apart again, but a glued joint is meant to stay together for a hundred years.

Key words

Joint

The place where two pieces of wood are connected, and the way they are shaped to fit.

Mortise and tenon

A joint where a tongue on one piece (the tenon) fits a pocket in the other (the mortise).

Dowel

A short round wooden peg glued into matching holes in two pieces.

Grain

The lines in wood. They show which way the tree's fibers run.

Finish

A protective coat, such as oil, wax, varnish or paint, put on wood to protect it and make it look good.

Kerf

The narrow strip of wood that a saw blade turns into sawdust as it cuts.

Check your thinking

1. Why is a mortise and tenon stronger than a butt joint?

   Show the answer

   The tongue locks inside the leg and gives the glue more to hold on to. The pieces lock into each other, so the wood itself carries the load, and there is much more glued surface.

2. Why do woodworkers say "measure twice, cut once"?

   Show the answer

   Because a piece cut too short cannot be made longer. You can always trim a long piece, but a short piece is wasted. Checking first saves wood.

3. What does a finish such as varnish do for a wooden stool?

   Show the answer

   It keeps water and dirt out of the wood. A finish seals the surface, so spills wipe off instead of soaking in and staining.

4. What are clamps for when a stool is glued together?

   Show the answer

   They squeeze the joints tight while the glue dries. Glue needs the pieces pressed together and held still until it hardens.

5. A wide solid-wood table top is glued firmly to its frame all the way around. What is likely to happen over a year of damp summers and dry winters?

   Show the answer

   The top may crack, because it cannot shrink and swell across its grain. Wood swells and shrinks across the grain as humidity changes. If it is held rigidly, the stress can split it, so tops are fixed with fasteners that allow movement.