Butt joining is an assembly method for the armor parts. You butt join the legs, not join the legs to the butt Instead of having two pieces that overlap with glue in the middle, you have two pieces butt up against each other, then use a cover strip on the front and back to hold them together.
Look here in the TK information post: http://www.whitearmor.net/forum/tutorials/article/22-howto-assemble-thighs-butt-join-with-cover-strips/