The vertices in directed graph families are so proud with the number of arrow going out of these vertices and the number of arrow going into the vertices. They make name degree of vertices to show their prestige.
To distinguish those degrees, they give name in-degree to count number of arrow going in to a vertex, and they give name out-degree to count number of arrow going out of a vertex.
Vertex r above has 1 in-degree and 1 out-degree.
Vertex p below has 1 out-degree and 0 in-degree. Vertex q, on the other hand, has 1 in-degree and 0 out-degree.
Vertices a, b and c in the graph below have 1 in-degree and 1 out-degree.
Vertex x has 2 out-degree and 1 in-degree, vertex z have 1 in-degree and 1 out-degree. Can you guess what is in-degree and out-degree of vertex y?
Poor vertex that has no arrow is called an isolated vertex because it has only zero degree .
These tutorial is copyrighted.
Preferable reference for this tutorial is
Teknomo, Kardi (2015) Pictorial Introduction to Graph Theory. https:\\people.revoledu.com\kardi\ tutorial\GraphTheory\