They're great on old guitars that were built before steel strings came into play, or on any guitar that is lacking proper bracing. Perhaps another rule of thumb is using in place of silk and steel. They sound great on guitars but keep in mind the punch or brightness compared to stantard strings just wont be there.If you are looking for soft mellow sustain, I highly recomend them. I used them for years on an old Bruno guitar where I was afraid to use steel.I even went as far as stringing up an old 5 stg banjo. Beautiful sound but no punch. Not recommended for performance with other instruments. The volume just isn't there.The strings are terrific for any parlor instrument where you want to keep the sounds quiet but still high quality. Spendy little critters. Jack the mostly folk guy