Histograms are a way of presenting information about the relative frequency of different values of a particular variable. The horizontal axis shows the range of that variable; it is divided into a number of bins -- successive intervals of the value, so a particular observation will fall in one of the bins like letters into a bin in the post office. The vertical axis represents how many times a particular value of the variable has been observed to fall in a particular bin. Cases where the value of the variable is below the low edge of the lowest bin are "underflows", and when the value is higher than the high edge of the highest bin it is an "overflow". These and the number of cases that fall in the range of the plot are registered in the under/in/over counters.