Line-chart based on a histogram, it is drawn by joining the mid-points of the blocks at their apexes with a straight line. The extreme points of the line are joined to the horizontal ('X') axis (where the mid-point of the respective next class would have been) to result into a polygon. In a very large set of data (where the number of classes increases and difference between their widths decreases) the polygon turns into a smooth curve known as frequency curve or frequency distribution curve. Histographs (and histograms) are commonly used where the subject item is discrete (such as the number of students in a school) instead of being continuous (such as the variations in their heights). Also called frequency polygon, a histograph is usually preferred over histogram where the number of classes is eight or more.