I have bought many and used a lot of these sets. High strings are made of a plastic blend patented by Aquila, with a yellowish color similar to real gut. If you look at them on a microscope you can see a rough surface due to rectification but the most interesting point is analyze a section of a string. You will see a fibrous structure similar to natural fibers, very different from Nylon. The bass strings are made of a multifilament red Rayon (synthetic silk) wound with a thin silver plated copper wire. I have to say these are not easy strings, they are not for every taste and for every guitar. The sound is clear but soft, you can play with a lot of expressivity. These high strings totally luck the commercial and shining sound of Nylon. When you first listen to these strings they appear weird, but when you familiarize with them and with the sound you will fall in love and if you turn back to Nylon, you will suddenly notice its commercial and plastic sound, with almost metallic nuances. The overall sonic footprint of bass strings is opaque, hot, they are metal wire wounded but you don't hear so much metallic profile. Basses are soft under your fingers so you can play with great interpretation and feelings. These strings are perfect to play music of the past like baroque and 1800's repertoire. The difference when listening to these strings compared to Nylon ones is just like the difference between the musicality and natural sound of valves amplifiers compared to solid state amps. High strings last for a long time but basses degrade pretty soon on the contact point with guitar metallic frets. If you play one-two hour a day you have to replace them after a month. Anyway, for this price it is not a big problem. They are the unique and true alternative to natural gut and the only strings you have to use if you are looking for a different sound than Nylon. Aquila knows hot to design and produce strings and they are among the very few producers who offer a personal product. Beware of every other brand, they sell different colored strings envelopes but inside them you have identical strings made of generic industrial purpose Nylon. Give yourself a chance and the time to carefully familiarize with these strings, your music will thank you.