To splice crown moulding, you will need a mitre saw or box, plus a hand or power saw.
Sometimes it may be neccessary to splice crown mouldings together on a long wall.
(A) Position the crown moulding pieces in mitre box and mitre the joining ends at 45° angles, keeping the mitre saw in the same position for both cuts.
This will allow overlap, creating a clean, precise, diagonal joint. For a secure fit between the joints, the two crown pieces can be nailed into a top stud or plate. Glue also assures a tight splice.