Here are a couple:
http://www.woodsmith.com/issues/165/videos/mitered-spline-jig/
http://www.woodworkingtips.com/etips/etip010511sn.html
http://www.ibiblio.org/twa/plans/plans/keymiterjig.pdf
For doing keyed mitered joints on boxes rather that frames. Make a similar jig only use a 2 x 4 or 2 x 6's instead.
corey
http://www.woodsmith.com/issues/165/videos/mitered-spline-jig/
http://www.woodworkingtips.com/etips/etip010511sn.html
http://www.ibiblio.org/twa/plans/plans/keymiterjig.pdf
For doing keyed mitered joints on boxes rather that frames. Make a similar jig only use a 2 x 4 or 2 x 6's instead.
corey