APA Dictionary of Psychology

a variation of a stem-and-leaf plot in which the trailing digits (leaves) from two data sets are displayed on either side of the same central column of initial values (stem). The leaf values from one data set are given on the right-hand side while those of the second group are shown on the left-hand side. For example, consider the following hypothetical sets of test scores obtained by participants on two different occasions: 79 79 83 83 86 87 92 99
and 80 81 82 83 87 87 90 99
The back-to-back stem-and-leaf plot for these values is
\begin{matrix} & | & 7 & | & 99 \\ 012377 & | & 8 & | & 3367 \\ 09 & | & 9 & | & 29 \\ \end{matrix}