The technical note described the origins of the 50 cm threshold used in Scotland to define a peat soil. These soils are required to have more than 35% organic carbon in the layers from the soil surface to at least 50 cm depth and is unique in the definition of peat soils. The origin of this depth threshold arose from work in the 1940s and 1950s by the Scottish Peat Committee, a government body that evaluated peat deposits for commercial use and was adopted by the Scottish Peat Survey and later by the Soil Survey of Scotland in the late 1960s/early 1970s. This report was funded by the Rural & Environment Science & Analytical Services Division of the Scottish Government. NB, the url of the reference 'Soil Survey of Scotland Staff. 2013. Unified soil classification of Scotland’s soils.https://www.hutton.ac.uk/learning/soilshutton/soil-classification. Accessed 14/04/2023.' has been updated to https://www.hutton.ac.uk/soil-maps/ (18/05/2025)