Back-To-Back Stemplot vs Stemplot
Both terms below come up in the same part of the course, and students mix them up. Here is each one defined on its own, side by side, so you can see where they part company.
Back-to-back stemplot
Graphs and displays
A back-to-back stemplot compares two groups on one shared column of stems, with one group's leaves read leftward and the other group's rightward.
A back-to-back stemplot is two stemplots sharing a spine, so the two shapes sit against each other and you can see at once which group runs higher. For example, on a stem of 6 that holds the tens digit, class A leaves of 5 and 8, read leftward away from the stem, are 65 and 68, while class B leaves of 1 and 3 read rightward as 61 and 63. No values are lost in the display, so each group's median can still be counted straight off the plot. The AP Statistics course framework lists back-to-back stem-and-leaf plots among the displays used to compare distributions of one quantitative variable.
Stemplot
Graphs and displays
A stemplot, or stem-and-leaf plot, displays quantitative data by splitting each value into a leading stem and a trailing leaf digit.
In a stemplot each value is split so the stem holds the higher place values and the leaf holds the last digit. For example, the values 21, 24, and 35 give a stem of 2 with leaves 1 and 4, and a stem of 3 with leaf 5. Reading the leaves off each stem recovers the original data, so no information is lost. Stemplots suit small data sets and make the shape easy to see.