A box-and-whisker plot shows the variability of a data set along a number line using the least value, the greatest value, and the quartiles of the data. Quartiles divide the data set into four equal parts. The median (second quartile, Q2) divides the data set into two halves. The median of the lower half is the first quartile, Q1. The median of the upper half is the third quartile, Q3.