Hello Diane, and thank you for your recent inquiry regarding this item from Prime-Line Products. Unfortunately there is not one mortise lock that is standard to all patio door locking systems, which is what this is for... and there is no sure way to determine the correct replacement lock without removing one and matching its dimensions to those listed online. Patio door manufacturers can often use more than one design within a given series or product line used over the years, making it sometimes difficult to provide an exact match. I would encourage you to visit Prime-Line’s website to find the correct replacement lock your door requires.
For additional questions regarding this item, or any other product manufactured by Prime-Line Products, please call their toll free number at (877) 649-9740, or contact them by email at email@example.com