PIERNIK is an MHD code created at Centre for Astronomy, Nicolaus Copernicus University. Current version uses a simple, conservative numerical scheme, which is known as Relaxing TVD scheme (RTVD). General mathematical context of the relaxation and relaxing systems of hyperbolic conservation laws, and related numerical schemes, was presented by Jin & Xin (1995). A particular realization of the RTVD was developed by Trac & Pen (2003) and Pen et al. (2003), who presented the numerical method in a pedagogical way, and provided short, publicly available HD and MHD codes. These codes rely on a dimensionally split, second order algorithm in space and time. The Relaxing TVD scheme is easily extendible to account for additional fluid components: multiple fluids, dust, cosmic rays, self-gravity.