Specifying the quality of software products is a valuable addition to functional specification, clarifying product properties such as learnability and availability. Specifying such properties is considered difficult due to the different parties involved and the implicit nature of the requirements. The QUINT project gathered experience with product specification by means of the Extended ISO model: an extension to the ISO 9126 model of software quality. By defining indicators and specifying how they should be measured, quality specifications can make requirements explicit. Recommendations and pitfalls for composing a specification are grouped by the context in which quality specifications can be used.