Martin M5XX Phosphor Bronze Acoustic Guitar Stringsstar rating 5ScottJuly 10, 2011I use these in 13 on my Martin D-18 Authentic and find they really punch through on bluegrass leads and is really punchy when you need it for rhythm accents. I love the open sound you get with a non-coated string and these last me a good 2 weeks playing 2 - 3 hours a day. Fantastic value.