-variables are described below. They are based on a standard filesystem
-layout; variants of it are used in SVR4, 4.4BSD, GNU/Linux, Ultrix v4,
-and other modern operating systems.
+variables and the values they should have in GNU packages are
+described below. They are based on a standard file system layout;
+variants of it are used in GNU/Linux and other modern operating
+systems.
+
+Installers are expected to override these values when calling
+@command{make} (e.g., @kbd{make prefix=/usr install} or
+@command{configure} (e.g., @kbd{configure --prefix=/usr}). GNU
+packages should not try to guess which value should be appropriate for
+these variables on the system they are being installed onto: use the
+default settings specified here so that all GNU packages behave
+identically, allowing the installer to achieve any desired layout.