On Tue, 12 Aug 2003 00:18:08 GMT, "Rural Knight"
wrote:

It's an expression related to combat (and other things) and
means to check behind you. Think of the face of a clock -
twelve would be high or in front (depending on plane), six
below or behind (depending on plane), etc.

It's a way to orient yourself in free space.
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Interesting, thanks.