How does gcc know where boost is installed

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http://unix.stackexchange.com – I installed boost using sudo apt-get install on Ubuntu. Later I did not have to give gcc the I flag alongwith the path to the boost libraries to use boost ! Nor did I do any pkg-config --cflags --libs of any sort. What is the mystery here ? How is gcc able to know where boost is and how is it linking to it automatically ? (HowTos)