The effect of deviations from ideal coincidence orientation relationship on the relative energy of a coincidence grain boundary was studied in tricrystals of high purity lead. Lower relative energies were observed within ±2° of the ideal 36.9° 〈100〉 coincidence relationship, with a maximum energy decrease of about 30 per cent. The present results, combined with previous energy data for different high-density coincidence boundaries in high purity lead, are shown to be related to the calculated number of shared atom sites per unit area (in lattice units) of the grain boundary. These results provide experimental support for the boundary coincidence and “relaxed” coincidence models.