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Two possible joints between different types of carbon tubules are discussed. Both joints introduce a single pentagon-heptagon pair into the perfect hexagon bonding pattern of the two tubules. These two types of joints can be used to form composite tubules in which every carbon atom is threefold coordinated. The first type of joint removes or adds a single hexagon from the circumference of the tubule and the second joins specific ``crenelated'' and ``sawtooth'' tubules of significantly different radii at an angle of 30^.
Brett I. Dunlap (1992) studied this question.