The term `angle' in the international system of units

The concept of an angle is one that often causes difficulties in metrology. These are partly caused by a confusing mixture of several mathematical terms, partly by real mathematical difficulties and finally by imprecise terminology. The purpose of this publication is to clarify misunderstandings and to explain why strict terminology is important. It will also be shown that most misunderstandings regarding the `radian’ can be avoided if some simple rules are obeyed.