Zieris, FranzPrechelt, LutzKoziolek, AnneSchaefer, InaSeidl, Christoph2020-12-172020-12-172021978-3-88579-704-3https://dl.gi.de/handle/20.500.12116/34545Background: Despite a lot of research on the effectiveness of Pair Programming (PP), the question when it is useful or less useful remains unsettled. Method: We analyze recordings of many industrial PP sessions with Grounded Theory Methodology and build on prior work that identified various phenomena related to within-session knowledge build-up and transfer. We validate our findings with practitioners. Result: We identify two fundamentally different types of required knowledge and explain how different constellations of knowledge gaps in these two respects lead to different session dynamics. Gaps in project-specific systems knowledge are more hampering than gaps in general programming knowledge and are dealt with first and foremost in a PP session. Conclusion: Partner constellations with complementary knowledge make PP a particularly effective practice. In PP sessions, differences in system understanding are more important than differences in general software development knowledge.enPair ProgrammingQualitative ResearchGrounded Theory MethodologyText/ConferencePaper10.18420/SE2021_481617-5468