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  "body": "\n## Definition\n\nA rooted tree is a pair $(T, r)$ of a [[tree|0MJ0JS72DZ]] $T$ and a vertex $r$ of $T$, the root.\n\nLet $v \\neq r$ be a vertex, and let $u$ be the neighbour of $v$ on the unique path from $v$ to $r$.\nThen $u$ is the parent of $v$, and $v$ is a child of $u$.\nThe depth of $v$ is the [[distance|0WPZ5YPAGR]] from $r$ to $v$.\n\n## Remarks\n\nThe root has no parent, and every other vertex has exactly one parent.\nA vertex with no child is a leaf of the rooted tree. With this convention the root can be a leaf only when the tree has one vertex.\nAn ordered rooted tree also fixes an order of the children of each vertex. A [[binary tree|02A8EMC83C]] is a related recursive structure.\n\n## Explanations\n\n### L1 human\n\nA rooted tree is a tree in which one vertex is chosen as the top.\nThen each edge has a direction: from a parent toward its child, away from the root.\n",
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