1. ‘International Classification of Hard Coals by Type’ Economic Commission for Europe, E/ECE/247, Geneva, 1956.
2. ‘The Coal Classification System Used by the National Coal Board (Revision of 1964)’ The National Coal Board, London, 1964.
3. ‘Classification of Coals by Rank’ Amer. Soc. for Testing Materials ANSI/ASTM D388–77, 1977.