I've done this a few ways.
I have seen this done even when the poster wasn't too large to be accomplished with standard 32X40 mats, The reasoning was the work ( N.O.Jazz and Heritage poster) was divided diagonally in the background coloration's. So it looked rather nice to do the same thing with the matting. However when the same poster had Remarque on it it did then become too large for standard mats. So an additional type of splicing extended the sides.
I first cut the regular mats as if they would be solid and drew matching diagonal lines on the reverse of both colors. next I'd cut them in opposing directions( one right beveled and one left beveled ) so when they were adhered together ( your choice of methods , and Hugh's tip may be best) there was very little or no visible seam for the standard size matting.
However with the remarque there was also a diagonal strip of Musical Score sheet at the joining point, so I cut small strips of Black core white mat to match the proportions of the score sheet and attached them at the joint of the two colors , cutting off the sides to match the boarders of the already cut mats. Then when the white strips where adhered together but not attached to the other mats I'd use a Dexter hand cutter to cut small Musical note in the strips to look like sheet music . Next I either placed desired mat cover paper under the openings or if too small just did a peal of the white to expose the black core to form the Notes. Now I attached the segmented white strips which increased the length of the sides of the standard mats to accommodate the over sized remarque.
I used the Black V-grooves on the sheet music because the multiple seems probaly would make the splices more evident and as such it would be better not to try to hide them.
However some very involved splicing of a different kind that I did can be seen if the images of the Kente Cloth 3D double bevel on Congo square /Jazz fest poster still are in the archives. The Inlays of the geometric patterns in the 1 inch wide strip are all done by splicing tiny segments of mats and adhering them together to form a strip which is underlay ed in the double bevel to match the kente Cloth on the Poster.
BUDDY
PS
Please excuse two things ,I didn't remember I had posted all of this before and it is in ARCHIVES, Next I can't seem to get the Images to appear even though the text is there to show what I redundantly am boaring Y'all about .
http://www.thegrumble.com/showthread.php?t=6819&highlight=1993+Jazz+Poster+PICTURES
Any suggestions would be helpful as to type of splice. Bevel to bevel, bevel to inverted bevel or anything else that has been successful. Considering I have purchased the mat stock already, I will stick to matting as opposed to fabric.
