Five A4 sheets of transparent acetate film. At one time used in offices everywhere to create business presentations for overhead projection, but now becoming harder to find (and very expensive).
We use acetate sheets principally for "gridding-up", a technique that helps with copying and scaling-up images. Draw a grid of equally-sized squares with a fine pencil or pastel onto your paper or canvas. Attach a sheet of acetate to the original image with a piece of tape and draw the same grid of squares on the acetate with a ruler and smear-proof fine-liner. If you are just copying the image, the squares will be identical on the acetate and the canvas/paper. If you are enlarging the image, the number of squares should be the same, but the size will be smaller on the acetate and larger on the canvas/paper. Now copy the lines from the image to the canvas/paper, one square at a time. It's much easier to keep the proportions correct when the image is broken down into smaller squares. For more help YouTube has some good videos. See School of Yule: "How to use the grid method with a drawing of Chuck Close as an example".
Acetate sheets can also be used for making stencils.