The invention pertains to a natural-variant combinatorial library of fibronectin Type 3 domain (Fn3) polypeptides useful in screening for the presence of one or more polypeptides having a selected binding or enzymatic activity. The library polypeptides include (a) regions A, AB, B, C, CD, D, E, EF, F, and G having wildtype amino acid sequences of a selected native fibronectin Type 3 polypeptide or polypeptides, and (b) loop regions AB, CD, and EF having selected lengths (Bottom Loops). The Fn3 may also have loop regions BC, DE, and FG having wildtype amino acid sequences, having selected lengths, or mutagenized amino acid sequences (Top Loops).