As you can see from the extended sine and cosine graphs, each curve repeats itself regularly. This trait is called "periodicity", because there is a "period" over which the curve repeats itself over and over. The length of the period for the sine and cosine curves is clearly 2π : "once around" a circle. Also, each of sine and cosine vary back and forth between –1 and +1 . The curves go one unit above and below their midlines (here, the x -axis). This value of " 1 " is called the "amplitude".