C1S refers to a paper stock that is coated only on one (1) side, versus C2S where the paper is coated on both sides. With C1S, the backside is completely uncoated, providing an excellent surface if a lot of writing is intended on that side. In the C1S family, we only sell 15pt C1S and all the rest of our gloss cover stocks are all C2S such as our: 14pt gloss cardstock. 16pt gloss cardstock. 24pt gloss cardstock.