Since tetrahedral chains always have a choice of 3 triangles where the next tetrahedron will be attached, they can be described by words on the 3-letter alphabet, {-. 0, +}. On a 3-letter alphabet, up to reversals and cyclic rotations, there are only twenty-one 4-letter words. To be a helix, a tetrahedral chain must have a dominant twist. Arbitrarily choosing to prefer a '+' twist, the 4-letter words with the number of '+' greater than 0 and greater than the number of '-' are only eight: '++++', '-+++', '000+', '00++', '0+0+', '0+++', '-0++', '-+0+':
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