Martin MA550FX Authentic Acoustic Flexible Core Medium 6-String Guitar Set

Martin uses the highest tensile-strength core wire, then tin-plates it on all six strings for added corrosion resistance; because what you don’t see matters! Coupled with high quality wrap wire, you get consistent true tone that you can count on song after song. With Flexible Core strings, you get total flexibility and total control. 
Phosphor Bronze — Medium Gauge .013 .017 .026w .035w .045w .056w
Flexible Core strings feel soft to the touch, making them ideal for beginners. But don’t be fooled by comfort! They are for professional players too. Just ask Tommy Emmanuel, who performs with Flexible Core strings night after night.