The standardisation of interfaces in product architectures helps complementary products develop when network externalities are present. However, standardisation may also weaken a technology developer's competitive position when the product knowledge embedded in standardised interfaces becomes accessible, thereby reducing the barriers to entry. Hence, there is a need to simultaneously protect the knowledge that underpins a firm's competitiveness, but also to define the standards that are open to encourage the development of complementary products. In this paper, we analyse different types and levels of knowledge that underpin a product. We apply this analysis to understanding how Nokia and Ericsson maintained their competitive positions during the Global System for Mobile (GSM)-dominated phase of the industry, even though they were instrumental in developing GSM as an entirely open standard.