English: Structural difference between a linear and a two-dimensional (2D) polymer. In the former, linearly connecting monomers result in a thread-like linear polymer, while in the latter laterally connecting monomers result in a sheet-like 2DP with regularly tessellated repeat units (here of square geometry). The repeat units are marked in red, whereby the number n describes the degree of polymerization. While a linear polymer has two end groups, a 2DP has an infinite number of end groups that are positioned all along the sheet edges (green arrows).