24The E.coli ribosome exit tunnel can accommodate small folded proteins, while 25 larger ones fold outside. It remains unclear, however, to what extent the 26 geometry of the tunnel influences protein folding. Here, using E. coli ribosomes 27 with deletions in loops in proteins uL23 and uL24 that protrude into the tunnel, 28we investigate how tunnel geometry determines where proteins of different 29 sizes fold. We find that a 29-residue zinc-finger domain normally folding close to 30 the uL23 loop folds deeper in the tunnel in uL23 Dloop ribosomes, while two 31 ~100-residue protein normally folding close to the uL24 loop near the tunnel 32 exit port fold at deeper locations in uL24 Dloop ribosomes, in good agreement 33 with results obtained by coarse-grained molecular dynamics simulations. This 34 supports the idea that cotranslational folding commences once a protein domain 35 reaches a location in the exit tunnel where there is sufficient space to house the 36 folded structure. 37