Measuring sticks and art grids are used for understanding form and checking layout when drawing a subject in front of you such as a person, static object or scene.

The measuring stick mainly provides height, size and other ratios for the objects in your painting.
Normally, it will be 30 to 50 centimeters long with graduations marked on a brass, straight, rectangular rod. A piano wire is attached to the tip of the stick by a screw and is used to measure angles and vertical and horizontal symmetry. However, anything can be used as a measuring stick as long as it is a straight rod. A bicycle spoke or a long pencil would do just fine too. For example, if you are measuring the size relationship between an apple and bottle, from the position in which you are working, stretch your arm out towards the subject holding the measuring stick. With the rod vertical, the distance between your eyes and the rod will remain constant. Close one eye and move the vertical stick so that the tip is lined up with the top of the apple. Next, while retaining the position, raise the thumb on the hand holding the stick so that it is vertical and work it up the stick so that the tip of your thumb is lined up with the bottom of the apple. The distance between the tip of the stick and the tip of your thumb is the benchmark space (height of the apple) for measuring. Now, line up the tip of the stick with the top of the bottle and count the number of benchmark spaces there are vertically down to the bottom of the bottle. This will show you how many times the height of the apple will fit into the height of the bottle. This method will allow you to check the relative sizes of a variety of subjects.

The art grid is a board with a small window which is used for deciding where to position a subject in a painting or drawing, and checking the relative positioning. The normal art grid comprises a black frame on a transparent plastic panel divided into 16 grids. Measuring approximately 115 x 140 millimeters, the inner dimensions of the black frame are compatible with specifications for charcoal paper and the Japanese B-series paper, etc., with different ratios for the long and short sides. It is generally used by holding the grid up over the subject and moving it around to find the desired layout, with the black frame representing the edges of your canvas. The artist can also draw a grid on a blank canvas and, after working out the layout, translate to the canvas while checking the lines that overlay the subject in the art grid with the lines on the canvas.

It is actually quite difficult to use these tools in the same posture and position and obtain accurate measurements so they should be used as a rough guide only.

Measuring sticks and art grids can be purchased at general art stores.