I own a '84 Martin D-12-28 and a '03 Martin J12-16GT guitars. This has to be the worst capo on the market for a 12 string guitar especially Martin 12 string guitars because of their wider necks. What I noted about this capo is that it notoriously knocks the guitar out of tune 9 out of 10 times. You cannot have that performing live. Absolutely not. I would not recommend this to anyone especially Martin owners to not buy this. Spring for the G7 guitar capo instead. More pricey? Yes. But well worth the money spent.