On Fri, 9 Oct 2009 15:30:00 -0400, "Frank K."
wrote:

He wants to cut slots through the center of the posts (3 slots) that are
just large enough to slide the boards through. There are going to be
lots of posts - maybe 100 or more.

How would you go about forming these slots? What sort of tool?

A little square holed wooden jig and a router bit with a collar is
what I'd use. Then, maybe a jigsaw or a sharp chisel to square the
inside corners. The other alternative if you can get the use of one,
is a mortiser.