Pupils begin the study of trigonometry, however brief, intensive, or extensive the course, and, whatever the grade in which it is taught, with the definitions of the six trigonometric ratios or functions. Much emphasis is put on these functions. Immediate applications to practical problems are made of the simpler ratios, such as sin 30°, tan 45°, cos 60°, and cot 135°. Shortly, the pupil is introduced to a table of “natural” functions, as we designate them to distinguish them from logarithmic trigonometric functions. The pupil learns to read the tables. He learns to apply them to problems that are not too difficult. It becomes an interesting game to use them. However, in his eagerness to make them serve him in solving problems of many sorts, whether he is a ninth-, tenth-, eleventh-, or twelfth-grade high school pupil, he loses the realization that he is using a ratio in decimal form. Somehow, to him, ratios are common fractions, and common fractions, ratios, but not so decimal fractions. The denominator of the decimal fraction is not apparent to him. Nor is it to us teachers until we make ourselves deliberately conscious of it. In truth, it is for this very reason that we use a decimal fraction, when we prefer it to a common fraction. It is due to this very fact that decimal fractions were invented.