In mathematics, S2S is the monadic second order theory of the infinite complete binary tree. S2S may also refer to: Server-to-server, protocol exchange between servers Site-to-site VPN S2S Pte Ltd, a Japanese record label Ski to Sea Race, a race in Whatcom County, Washington Sister2Sister, Christine and Sharon Muscat, Maltese-Australian singers Sales to Support, type of transfer on Call Center lines, Sales to Technical Support Transfer In woodworking or lumber terms S2S= surfaced two sides, while S3S = surfaced three sides and s4s = surfaced four sides