The essential features of lettering on technical drawings are, legibility, uniformity and suitability for microfilming and other photographic reproductions. In order to meet these requirements, the characters are to be clearly distinguishable from each other in order to avoid any confusion between them, even in the case of slight mutilations. The reproductions require the distance between two adjacent lines or the space between letters to be at least equal to twice the line thickness. The line thickness for lower case and capital letters shall be the same in order to facilitate lettering.
The following specifications are given for the dimensions of letters and numerals:
- The height of capital letters is taken as the base of dimensioning.
- The two standard ratios for d/h, 1/14 and 1/10 are the most economical, as they result in a minimum number of line thicknesses.
- The lettering may be inclined at 15° to the right, or may be vertical.
NOTE: The spacing between two characters may be reduced by half, if this gives a better viusal effect as for example LA, TV; it then equals the line thickness.
Figures “Inclined lettering” and “Vertical lettering” show the specimen letters of type A, inclined and vertical and are given only as a guide to illustrate the principles mentioned above.