The formal text of the International Temperature Scale of 1990 (ITS-90) gives a concise definition of temperatures T90 and is highly prescriptive. To accompany the formal text, supplementary information concerning the practical realization of the scale was published by the BIPM in a monograph called Supplementary Information for the ITS-90; this monograph provided the non-prescriptive details in a comprehensive form.
The initial edition (dated 1990) is currently being updated to take account of the many advances in thermometry that have been achieved since then, concerning both temperature fixed points and interpolation instruments. The new edition is being written and prepared by the CCT Task Group for the Realization of the Kelvin (CCT-TG-K) and each chapter is approved by the CCT.
This new edition, Guide to the Realization of the ITS-90, is published uniquely in electronic form on the BIPM website. The chapters are self-contained but harmonized documents allowing a higher flexibility for future revisions to follow the pace of new developments. The presented material could be assembled only thanks to the continual assistance of many colleagues in the temperature-measurement community.
There are many sources of information, which requires a subjective selection of the references, and unintentional errors of omission are certainly inevitable; comments and proposals for future revisions delivered via the CCT are most welcome.
For those parts for which a revision is not yet available (no link below) the 1997 reprinting of the 1990 edition still presents useful information.