Straight and Right Angles

• A straight angle is nothing more than a straight line that measures exactly 180 degrees. A full circle measures 360 degrees and since a straight angle cuts a circle in half, it equals 180 degrees. A right angle is an angle that makes a perfect L shape. It measures 90 degrees and takes up one fourth of a circle. One fourth of 360 degrees is 90 degrees.
  
  

Acute and Obtuse Angles

• An acute angle is any angle that has a measurement of more than zero degrees but less than 90 degrees. For example, an angle measuring 25 degrees is acute because it is between zero and 90 degrees. An obtuse angle measures more than 90 degrees but less than 180 degrees. For example, an angle measuring 135 degrees is obtuse. Many fourth-graders tell the difference between these angles by remembering that an acute angle is a cute little angle.
  
  

Lines, Line Segments and Rays

• A line is a straight pathway that does not end in either direction. It has an arrow on each end to show that it continues forever in both directions. A line segment is also a straight pathway, but it has two endpoints. In other words, it has a fixed length and does not continue in both directions. A ray has one endpoint, but the other end has an arrow to signify that it continues endlessly in one direction.
  
  

Parallel, Perpendicular and Intersecting Lines

• Parallel lines are two lines that continue endlessly in both directions next to each other. Neither line in the set ever crosses the other line's path. Fourth-graders usually learn this by referring to train tracks as parallel lines. Perpendicular lines are two or more lines that run into each other to form an L shape, or right angle. The capital letter T is one example of perpendicular lines because it forms two right angles. Intersecting lines are two or more lines that run into each other to form acute or obtuse angles. The letter X is one example of intersecting lines because it forms two obtuse angles and two acute angles. Rays and line segments can also be parallel, perpendicular and intersecting.