The two most important characteristics of opto-mechanical structures used in space or for astronomy are high accuracy and stability. Much time, money and energy is invested in achieving this. However, because resources are limited, it is important to realize that the cost of an instrument is mainly determined during the design process. Three factors can influence the design to a great extent. These are: creativity, design rules and analysis. The most important tool for analysis nowadays is the computer. This has become so powerful that even large structures are no problem to analyse in detail using finite elements. It is even tempting to spend much time and effort in optimising structures with the computer. However, basis for the best result is creativity during the design phase and the application of design rules. Although design rules are used extensively, it is surprising to see that one of the most important ones is so little used. This paper therefore deals specifically with the rule to make structures statically determined. The importance is invaluable because to apply this rule means much less need for computers. Furthermore its application often results in non-conventional structures and it helps to clearly define the basics of the structure. It also provides simple qualitative results that help to make decisions concerning alternatives. Results of computer models can be verified on their validity. Some interesting results of the application of this design rule are shown. Examples are the linear guiding system for an optical delay line and the design of a telescope. Also, existing structures, like the secondary mirror mount of the VLT, will be analysed and it will be shown what they would have looked like had they been statically determined.