Akiyoshi Wada retired some years ago as a professor of physics at Tokyo University, where he was for many years one of those in Japan urging the fuller use of physical techniques in molecular biology notably in the automatic sequencing of DNA. Now he is the director of the interdisciplinary Sagami Chemical Research Center near Tokyo. The Wadarules reproduced on these pages (together with an explanatory note by Professor Wada) do not arise directly from the author’s professional work, but are the product of a fertile and ingenious imagination exercised, in part no doubt, on long aircraft journeys. They are offered without restriction to those who may have use for them. It is simply necessary to photocopy the diagrams, to separate the main rule from the reference rule and to mount each part on stiff card in such a way as to be able to slide the latter against the former. While the Wadarules may have a general heuristic value for those who have the time to play with them, they mag have a particular educational value for young children and may be particularly useful for, say, science writers forever seeking graphic ways of dramatizing the large or small scale of some phenomenon (as in the phrase "seems no bigger than a dime at a distance of 1 mile"). The rules can readily be amended to include objects with dimensions other than those shown. The annotation of the time rule leaves ample room for the addition of extra items, such as the average duration of the human heart-beat, the period of oscillation of "middle C" (0.00178 seconds) and the like.