Product lines embody a strategic reuse of both intellectual effort and existing artefacts, such as software architectures and components. Third-party components are increasingly being used in product line based software engineering, in which case the integration is controlled by the product line architecture. However, the software integrators have difficulties in finding out the capabilities of components, because components are not documented in a standard way. Documentation is often the only way of assessing the applicability, credibility and quality of a third-party component. Our contribution is a standard documentation pattern for software components. The pattern provides guidelines and structure for component documentation and ensures the quality of documentation. The pattern has been validated by applying and analysing it in practice.