Principles describe changes in things. Those "things" are concepts which are simpler components of the principle and must be mastered before it is possible for a learner to master the principle. For example, the law of supply and demand describes relationships among changes in price, supply, and demand. Those three concepts must be mastered before the law of supply and demand can be mastered. 

Procedures describe how to change things (to achieve a goal). Those "things" are also concepts which are simpler components of the procedure and must be mastered before the procedure can be learned. For example, the procedure for dividing fractions indicates that the divisor must be inverted, the resulting numerators must be multiplied to form the new numerator, and the denominators must be multiplied to form the new denominator. The three conceptsódivisor, numerator, and denominatorómust be mastered before it is possible to master the procedure (although it is not necessary to learn those labels). Also, procedures often require the use of tools to perform a step, and those tools are concepts. 

Given the importance of concept classification, it is essential that we know how to teach it well. Fortunately, much research has been done in this area, so we understand much about how concept classification is learned and what features instruction should have to best help learners learn it.