His 1919 book, Geometrische Kristallographie des Diskontinuums, played a seminal role in the refinement of space group theory. In this book, Niggli demonstrated that although X-ray reflection conditions do not always uniquely determine the space group to which a crystal belongs, they do reveal a small number of possible space groups to which it could belong. Niggli used morphological methods to account for internal structure and, in his 1928 Kristallographische und Strukturtheoretische Grundbegriffe, he took up what is essentially the reverse process, the task of establishing the connection between space lattices and external crystal morphology.