The 26 capital letters of the alphabet can be topologically viewed as simple networks - collections of links joined by nodes. Nodes are classified by the number of links that meet there. A well-known theorem in graph theory states that a network contains as Euler Path (a path that traverses the network, once only along each link) if and only if it has at most two nodes with an odd number of links. (This theorem was once used by Euler to prove that one could not traverse the seven bridges of Konigsberg without repeating one or more of them.)
"Euler Path Words,"
Word Ways: Vol. 37
, Article 12.
Available at: http://digitalcommons.butler.edu/wordways/vol37/iss4/12