International Tables for Crystallography started life in in 1935 as a two-volume set entitled Internationale Tabellen zur Bestim- mung von Kristallstrukturen, with C. Hermann as editor. We are now in the third series, with eight volumes covering all aspects of crystallography from symmetry to macromolecular crystal- lography. However, there has always been one glaring omission and one that has become increasingly serious: powder diffraction. This is odd: powder crystallography started as early as 1916 with the seminal work of Debye and Scherrer, and has grown to include quantitative and semi-quantitative analysis, structure solution and refinement, two-dimensional data, comprehensive databases, clustering, and microstructural properties, and is applied to a wide range of problems of both academic and industrial interest. Articles in the International Union of Crys- tallography’s monthly Journal of Applied Crystallography are dominated by powder-diffraction papers. In terms of instru- mentation, there are more powder diffractometers ($10 000) in use worldwide than any other comparable diffraction instrument. There have also been rapid advances in radiation sources and detectors, and major developments in software, computing power and visualization tools, all of which have made what was once cutting-edge science commonplace.
The methodology that has done more than anything to trans- form the field treats the measured powder-diffraction data in a comparable way to single-crystal data (albeit with more restric- tive conditions) and is generally known as Rietveld refinement. This method will be found everywhere in this volume and was developed in the 1960s by Bert Loopstra (who came up with the concept), Bob van Laar (who worked out the mathematics) and Hugo Rietveld (who wrote the first computer program for it), as discussed in a recent article by van Laar & Schenk [Acta Cryst. (2018), A74, 88–92].