A detailed explanation of the art and science of dimensioning for existing or aspiring designers. Most creators of technical drawings do not appear to understand the basic principles of dimensioning, nor do they understand exactly what the tolerances collectively will or will not do. Equally, inside the workshop few trained and experienced engineers have a similar understanding. . The reason is that the subject is not taught in Universities, Technical Colleges, or Workshop Technology books, and where it is, it is done badly, as the people doing the teaching are themselves in ignorance. This work is an attempt to help designers and engineers improve their drawing standards. .