Described and claimed herein are combinatorial synthetic Fab libraries displayed on a phage pIX protein. The libraries were built on scaffolds representing the most frequently used genes in human antibodies, which were diversified to mirror the variability of natural antibodies. After selection using a diverse panel of proteins, numerous specific and high-affinity Fabs were isolated. By a process called in-line maturation the affinity of some antibodies was improved up to one hundred-fold yielding low pM binders suitable for in vivo use. This work thus demonstrates the feasibility of displaying complex Fab libraries as pIX-fusion proteins for antibody discovery and lays the foundations for studies on the structure-function relationship of antibodies.