Non-standard symbols for the tonne (metric ton), used by some engineers in the UK. The only proper symbol for the tonne is t. If it is necessary to distinguish the tonne from the British Imperial ton, use tn for the British unit
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}