Hi Bahrend, Thanks for your answers. Because I'm not sure I fully understood your clarifications, I decided to repeat your answers in my own words, so you can correct me if I misunderstood them. a) Do you agree that the documentation of the "geometry concepts" is currently insufficient, because both the intended semantic meaning and the exact constraints on valid "values" for objects modeling the "geometric concepts" are not documented? You agree. b) Why are there two examples that more or less contradict each other? Because you planned to introduce the concept of closeness at one time, but renounced on it for the review. c) Is there a bug in the area function, or are both "Modules" -> "A Generic Geometry Library (web-copy of article for BoostCon'09)" and "Example" -> "c04_b_custom_triangle_example.cpp" wrong? There is no bug in the area function. If the concept of closeness would have been introduced, both "..." and "..." would have been correct. The example just demonstrates that this would be possible. Regards, Thomas