It has been discovered that the formation of the butterfly twin in barium titanate depends on both the average particle size and the particle‐size distribution of the starting material. Maximum twin yields occur when the particles have an average size near 1μ with a rather sharp cutoff in the distribution in the large‐particle region. All impurities act to decrease twin yield, principally because they interfere with stacking‐fault formation which also is necessary for twinning to