As Count Korzybsky pointed out, language is the tool we think with, and a poor tool does poor work. For twenty-five years we have measured the x-ray dose in roentgens. The accuracy of the instruments available to measure it has led us at times into pedantry that compromised our sense of humor. Sometimes a patient comes, carrying a letter wherein it is stated that on March 2, 1953, he finished a course of treatments, 203 r per day to a total of 4,060 r, given at 197 kv. with 0.5 mm. Cu filter plus 1 mm. Al, h.v.l. 0.96 mm. Cu. Such precision! In his preoccupation with the third significant figure, the radiologist has forgotten to tell you the size of the field. If you are of a sarcastic turn of mind, which God forbid, you might write asking him whether treatment was at 50 cm. or 52. And whether the patient was treated on Sundays and on Washington's birthday. He wouldn't understand the rhetorical purpose of your demand for yet finer precision in the treatment details, and you would merely distract his attention from your only pertinent question, namely: were the roentgens of exposure measured in the bare x-ray beam, or does that 203 r “dose” refer to measurement made on the patient's skin? When you are measuring the output of your tube, a unit of exposure fits your needs perfectly. But when you are planning and recording treatments -when your attention is on the patient and his skin and his tumor—then you think of the dose almost as if the tissue were soaking up dye and becoming redder the stronger the dye and the longer the application. A unit of exposure does not fit this concept well, but we have been using our roentgen under an unconscious perversion of its defined meaning so as to fit it to this practical need. In fact, we see in the official glossaries a tendency to redefine “dose” so that the concept will fit the unit, since the unit does not fit the concept. Thinking becomes even muddier when to confusion of dose with exposure is added a further confusion of radiation dosage with drug dosage. Exposure is spread over an area. Dose is spread through a volume. The unit of volume is one cubic centimeter for radiation, but for a drug the unit is one patient. The confusion reveals itself most flagrantly when, for example, the letter of referral reports x-ray treatments of 12,000 r and it transpires that four fields were treated, 3,000 r to each. In 1950, the International Committee agreed that treatment should be recorded in terms of the significant quantity, namely the specific energy absorption, for which the fundamental physical unit is the erg per gram. In 1953 the Committee adopted a practical unit, 100 ergs per gram, and named it the rod. It is so nearly equal, numerically, to the 93 ergs of energy that 1 c.c. of water will absorb from an exposure to 1 r, that the conversion factor is hardly important.