Tree rings are commonly used proxy data for past climate variability. Probably the simplest practical solution for transforming raw tree-ring data into proxy estimates and retaining information on low-frequency tree-growth forcing is the regional curve standardization (RCS). This paper reviews the RCS concept and the development of this standardization method over the past 25 years. Tree-ring based estimation of low-frequency climate variability is illuminated with a growing diversification of the original concept. The RCS-type methods are seen to remain as essential tools in palaeoclimate research while the RCS chronologies of tree-ring and other incremental proxy data remain the only source for calendar year dated short-to-long timescale climate variations.