You are right; a good explanation of the rules is in the C FAQ [1], which points to a newsgroup posting by Kaz Kylehu [2]. I am posting the summary here, altho

You are right; a good explanation of the rules is in the C FAQ [1], which points to a newsgroup posting by Kaz Kylehu [2]. I am posting the summary here, although please do read the original if you have time: The most portable thing to do is to use "" for including files within your project, and to use <> only for implementation supplied files. (Disclosure: I was one of the contributors to the C FAQ). [1] https://c-faq.com/cpp/inclkinds.html https://c-faq.com/cpp/inclkinds.html [2] https://c-faq.com/cpp/inclk.kaz.html https://c-faq.com/cpp/inclk.kaz.html