All vertices in a closed trail have even degree.
The Attempt at a Solution
Intuitively, I know this statement is true, but I can't seem to see a clear way to show it. I know that a closed trail is a path that connects vertices, so one would follow an edge through a vertex to another edge, thus indicating that, on this particular path, this particular vertex has degree two. Clearly, a non-closed trail has endpoints which must have odd degree because they reach a stopping point on the trail, with a vertex incident to only one edge. Will you please help me put these facts into a coherent proof? Thank you.