If you ordered stickr seals with your wedding invitations, you traditionally place them on the back of the envelope, so the center of the seal is right at the point of the flap. In the days before envelopes with the glue already applied, not to mention stickers, this is the location wax was dripped and stamped with some sort of monogram or motif to hold the envelope closed. In many cases these days, stickers have replaced wax seals, but wax seals are sometimes still used on formal wedding invitations, just for that Victorian look.