Set in the middle the protractor is a red ‘positioning disk’, around which the ruler can rotate to any angle. ‘Centimetre’ measurements are indicated at the top, and the protractor measurements on the underside. Users press and hold the positioning disk while rotating the ruler to draw or measure the angles. Specific operating steps (corresponding to images 1 to 4): Step 1. Draw a line OA, place the underside of the ruler (marked with red) along the line, so that the desired axis point O corresponds with 30 mark on the ruler. Step 2. Press and hold the ‘positioning disk’, rotating the ruler until the top of the ruler (marked with black) corresponds with the axis point O; Step 3. Starting from O, draw a line along the ruler as OB; Step 4. A 30-degree angle is completed.