We present a growth model for single-walled carbon nanotubes (SWNTs) based on experimental observation and energetic considerations. Carbon rings, observed experimentally and suggested to be fullerene precursors, are the templates for SWNTs nucleation. SWNTs grow by addition of C-2 radicals to catalytically induced reactive sites. This model is consistent with known experimental results and explains the characteristic diameter distribution of SWNTs as resulting from the gas-phase distribution of carbon ring isomers.