Example 4.2.5.

Consider the tree below.

$A labeled tree. At the center of the drawing is the vertex e, adjacent to f (top right), g (bottom right), d (bottom left), c (middle left) and b (top left). Vertex c is also adjacent to vertex a to its left. Vertex f is adjacent to h (to its right) and i (to its down-right).$

If we designate vertex $f$ as the root, then $e\text{,}$ $h\text{,}$ and $i$ are the children of $f\text{,}$ and are siblings of each other. Among the other things we cay say are that $a$ is a child of $c\text{,}$ and a descendant of $f\text{.}$ The vertex $g$ is a descendant of $f\text{,}$ in fact, is a grandchild of $f\text{.}$ Vertices $g$ and $d$ are siblings, since they have the common parent $e\text{.}$

Notice how this changes if we pick a different vertex for the root. If $a$ is the root, then its lone child is $c\text{,}$ which also has only one child, namely $e\text{.}$ We would then have $f$ the child of $e$ (instead of the other way around), and $f$ is the descendant of $a\text{,}$ instead of the ancestor. $f$ and $g$ are now siblings.