decision trees and numeric attributes
Each vertex in a decision tree is associated with a question of the form "$x_i &lt; c$?" (or "$x_i \leq c$?"), where $x_i$ is one of the input and $c$ is a constant. The decision whether to go left or right then depends on which side of the hyperplane $x_i = c$ the input is located.

The possible hyperplanes which come up this way are not arbitrary – they are orthogonal to one of the axes; the hyperplane $x_i = c$ is orthogonal to the $x_i$ axis. An example of a different hyperplane, which cannot occur, is $x_1 - x_2 = 0$.