Box plots may also have lines extending from the boxes (whiskers) indicating variability outside the upper and lower quartiles, hence the terms box-and-whisker plot and box-and-whisker diagram.
Outliers may be plotted as individual points.
Box plots are non-parametric: they display variation in samples of a statistical population without making any assumptions of the underlying statistical distribution (though Tukey’s boxplot assumes symmetry for the whiskers and normality for their length).
Box plots can be drawn either horizontally or vertically. Box plots received their name from the box in the middle.