John:

Basically, it allows 8 mil trace and 8 mil space, allowing one trace
between .050" pitch smt I.C.s, and two traces between 0.100" pitch
thru-hole devices using 0.060" lands.  This was pretty much the standard
when SMT was first becomming popular and there was conventional thru-hole
mixed with SOIC's.  It applies most importantly with autorouters that use
fixed grids, such as the old PADS rip-up and re-try autorouter.

Dave.
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