Crystal engineering techniques that mimic triple helical fragments in the crystal lattice are able to reveal interesting features of short triple Pu.PuPy helical fragments. The nonamer d(GCGAATTCG) and decamer d(GGCCAATTGG), containing respectively one and two overhanging guanines, were designed to form G center dot GC triplets in the crystal packing. To introduce a third subsequent T.AT triplet and thus a Pu.PuPy to Py.PuPy triplet transition, the decamer was further extended with one overhanging thymine residue. The resulting undecamer d(TGGCCTTAAGG) forms an extra parallel Hoogsteen T.AT triplet at one end of the duplex and an unexpected (swung out over 270 degrees), antiparallel reverse-Hoogsteen T center dot AT triplet at the other end of the duplex, providing detailed X-ray structural models of parallel and antiparallel T center dot AT triplets. The derived parameters allow for the construction of larger parallel and antiparallel triple helical models.