[Illustration:
+-------+ +----- A B | | / \ | | / |\ \ | | / /| | \ \ | | / / | | \ \| |/ / | | \ | / / | | \ |\ /| / | +-----\-|-\/-|-/-----+ \| /\ |/ |/ \/ |\ /\ /| \/ |\ +-----/-|-/\-|-\-----+ | / / \| \ | | / | \ \ | | / /| |\ \ | | / / | | \ \ | |/ / | | \ \| / | | \ / | | \ -----+ +-----
]
There are just sixteen points (all on the outside) where three roads may be said to join. These are called by mathematicians "odd nodes." There is a rule that tells us that in the case of a drawing like the present one, where there are sixteen odd nodes, it requires eight separate strokes or routes (that is, half as many as there are odd nodes) to complete it. As we have to produce as much as possible with only one of these eight strokes, it is clearly necessary to contrive that the seven strokes from odd node to odd node shall be as short as possible. Start at A and end at B, or go the reverse way.
