... simply means gauging proportions among elements. For example, the height of a chair may be twice as long as its width. We measure by determining a standard against which to compare proportions.

In this diagram from your textbook, you see that your pencil can be used to measure proportions. Hold the pencil so it appears to be aligned with an edge of the table. Place the tip of the pencil at the table's highest point and note the lowest point with your thumb on the pencil. Keeping your thumb in place, you can then compare the full height of the table to its full width (furthest point left to furthest point right).

Remember to measure from the same position and to keep your arm straight and elbow locked. Variations in the positioning of your body or arm will disrupt the consistency of your measurements.