The crystal orientation relationships and atomic arrangements across deformation twin planes and twin-related lamellar interfaces in [(1) over bar 10] projection directions in TiAl are examined crystallographically. Atomic arrangements across the true-twin plane and the true-twin-related lamellar interface are shown to be identical, while the atomic arrangements across the pseudo-twin plane and the pseudo-twin-related lamellar interface are quite different. Atom locations at the true-twin plane and the true-twin-related lamellar interface do not violate the atomic order in both crystals while those at the pseudo-twin plane or the pseudo-twin-related lamellar interface are shown to violate the atomic order. [(1) over bar 10] zone diffraction pattern simulations show that the true-twins and the true-twin-related lamellae cannot be distinguished using the [(1) over bar 10] zone diffraction patterns. Not every [(1) over bar 10] zone diffraction pattern can be utilized to distinguish between any two different twin relationships. The true-twin and the pseudo-twin can only be distinguished using [(1) over bar 10] zone diffraction patterns, while true-twin-related and pseudo-twin-related lamellae are distinguishable by only using [(1) over bar 10] zone diffraction patterns.