A steel rule is the simplest and most common measuring tool. The flat steel rule is usually 6 or 12 inches long, but longer sizes are available. Steel rules can be flexible or nonflexible, thin or wide. The thinner the rule, the more accurately it measures, because the division marks are closer to the work.
Generally, a steel rule has four sets of marks, two on each side of the rule. On one side are the inch marks. The longest lines are for 1-inch increments. On one edge of that side, each inch is divided into eight equal spaces of 1/8 inch each. On the other edge of that side, each inch is divided into 1/16-inch spaces. To make counting easier, the 1/4 inch and the 1/2-inch marks are normally longer than the smaller division marks. The other side of the steel rule is divided into 32 and 64 spaces to the inch. Each fourth division in the inch is usually numbered for easier reading.