Abbreviations for the Latin diebus alternis, every other day, and diebus tertius, every third day, units of frequency sometimes used in medical prescriptions
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}