## Elementary Geometry for College Students (5th Edition)

First, look at both angles. You will see that ABC has a narrower opening than DEF, meaning it is a smaller angle (the angle ABC will measure fewer degrees than DEF). Also, intuitively, you can see that angle ABC is narrower than the angle of a standard piece of paper (which is a right angle, also known as a 90$^{\circ}$ angle), which means it is an acute angle. DEF is wider than the angle of a piece of paper, so it is an obtuse angle.