A histogram is a bar chart in which data values are grouped together and put into different classes. These classes often present the frequency distributions found in the data set. Histogram coordinate systems are based on the horizontal axis and vertical axis. These two axes give the histogram the widths of the group that are equal to the class intervals and heights equal to the corresponding frequencies. Bars often represent the visual aspect of histograms; the height of each bar corresponds to its class frequency. As a result, the histogram makes the middle of the distribution visually apparent. Histograms based on relative frequencies show the proportion of scores in each interval rather than the number of scores. This entry discusses the history of histograms and how they are used with statistics, data, and probability distributions.