The unit of measurement you use to quantify the lengths of the 2 lines is irrelevant – just make sure you're using the same unit for both lines. Please help us continue to provide you with our trusted how-to guides and videos for free by whitelisting wikiHow on your ad blocker. If an angle is less than 90 degrees, it's an acute angle. In other words, if you measure the length of one line in centimeters, you should also measure the other in centimeters. How to Use a Protractor to Draw an Angle. The other arc begins with 0° on the left side. Let’s start. Labeling an angle acute or obtuse helps you reading the protractor. The horizontal line becomes the adjacent side of your triangle, and the vertical line becomes the opposite side of the acute angle you want to measure. % of people told us that this article helped them. If the second angle is larger than the first then the points M (and so S) will be below the line PQ. Once you have these measurements, divide rise by run to find the slope, or the steepness, of the diagonal line. Reflex: an angle greater than 180-degrees but less than 360-degrees. For example, if the reflex angle produces an acute angle of 18 degrees, that would mean the reflex angle is 198 degrees.