This paper's main issue is a strong advocacy in favour of an a priori classification of thermal environments that can be really functional to comfort assessment: Class 1, environments where comfort conditions can be established (comfort-prone environments), and Class 2, environments where this is not practically feasible. The former, which are also identified here as 'thermally unconstrained' environments, because of the absence of elements preventing comfort from being pursued, are the subject of a novel classification scheme. In assembling such a scheme, the four standardized synthetic indexes (Predicted Mean Vote, Insulation REQuired, Predicted Heat Strain, Wet-Bulb Globe Temperature) have been carefully scrutinized, with special emphasis on the regions of overlap. Additional data from national technical documents and legislation have been used to help in assembling the discomfort assessment scheme. All available information has been reprocessed and cast in a form specific for use in comfort-prone environments. Classification takes place through placement in a four-level and in a six-level discomfort scale for cold and warm environments, respectively; for each area, a recommended descriptor as well as a time frame for intervention are specified. The new scheme also eliminates a few glitches and inconsistencies existing in the ISO 15265 scheme, mostly in the area of cold discomfort. Being solely concerned with comfort-prone environments and keeping an open mind with respect to all available information, the new classification scheme represents a simple and robust all-round tool, tackling issues related to both comfort assessment and to action planning for an optimized allocation of available resources.