Before cutting these mortises I first had to fit the Heart tenons into the top of the Arch and the aforementioned top muntin rail. The top had to be cut to the exact length, tenoned into and coped over the quarter round of the Arch rail. You cannot cut it too short ! So I cut it too long - twice, to get it tight. After fitting it together I could now get the exact locations and lengths of the diagonal muntins. Each side of the muntins were unique to the curves of the Arch and the Heart - painstaking fitting. it took a full day to fit eight ends - tenons fitting tightly, sixteen copes - two per end - cut precisely. THAT was a good days work ! Here's some of the process. >