Reaction reversals to within 0.2 kbar locate the quartz⇄coesite equilibrium at 24.6, 24.9, 25.5, 26.2, 27.0, 27.9, 28.8, and 29.7 kbar at 350, 400, 500, 600, 700, 800, 900, and 1000¿C, respectively, using well-calibrated (frictionless). 2.54-cm diameter NaCl furnace assemblies in piston-cylinder apparatus. The equilibrium is unaffected by the presence of H2O or jadeite, contrary to previous proposals. The positve dP/dT of the reaction at T<500¿C is inconsistent with existing thermochemical data for coesite. Numerous tight reversals provide a sound basis for use of the quartz⇄coesite equilibrium as a calibration of pressure in high-pressure apparatus.