The structural features required for the formation of two- versus three-stranded coiled coils have been explored using de novo protein design. Peptides with leucine at the 'a' and 'd' positions of a coiled-coil (general sequence: Leua Xaab Xaac Leud Glue Xaaf Lysg) exist in a non-cooperative equilibrium between unstructured monomers and helical dimers and helical trimers. Substituting valine into each 'a' position produces peptides which still form trimers at high concentrations, whereas substitution of a single asparagine at the 'a' position of the third heptad yields a dimer. During the course of this work, we also re-investigated a helical propensity scale derived using a series of coiled-coil peptides previously believed to exist in a monomer-dimer equilibrium (O'Neil & DeGrado 1990). Detailed analysis of the concentration dependence of ellipticity at 222 nm reveals that they exist in a non-cooperative monomer-dimer-trimer equilibrium. However, the concentration of trimer near the midpoint of the concentration-dependent transition is small, so the previously determined values of delta delta G alpha using the approximate monomer-dimer scheme are indistinguishable from the values obtained employing the complete monomer-dimer-trimer equilibrium.