Am finishing my basement - have had it water-proofed, put a vapor
barrier on the walls, will insulate, etc. The problem I'm running
into is in the framing.

I have a ledge that runs around most of my basement (knee-wall/frost
wall??). It's 3 feet up and 23" wide.

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How do I frame this? Some ideas I've had involve a single top/bottom
plate with the 2X's cut at 45-degree angles - or joined at 90-degree
angles. I really don't want to have two top/bottom plates unless I
absolutely have to.