To solve this bridge problem Euler changed the picture into a graph.
 

This is the original picture of the problem.
 

Euler simplified the picture by labeling the islands A and D and the shores of the river are B and C. The bridges are a, b, c, d, e, f, and g.
 

Euler simplified his graph one more time.  In this graph the vertices represent the land regions and the edges represent the paths over the bridges.