Distances among teats were measured immediately prior to and immediately following milking in 912 lactations of 506 Holstein cows to assess variation due to age (parity), stage of lactation, year, and observer. Data were used to simulate success or failure to attach by the Gascoigne Melotte robotic milker. Distance between front teats premilking was nearly twice that for rear teats. When attachment of the cluster to an udder with a particular set of teat distances was simulated, excessive distance between front teats was the most frequent cause of attachment failure for third and later parities. For first parity, extremely narrow distance between rear teats was the most frequent cause of predicted attachment failure. Reduction in teat distances because of milking was proportionately much greater for distance between rear teats (45 to 50%) than for other teat distances (25 to 29%). In a study of 10 Holstein cows measured at milking and at 0, 6, 8, and 12 h after milking, distances among teats increased linearly with time elapsed postmilking. Electronic storage of a profile of a cow for teat distance and adjustments for milking interval and stage of lactation can increase chances for successful cluster attachment.