Other Makefiles can be included from a Makefile with the .INCLUDE directive (it must appear at the beginning of a line, like any other directive). For example,
.INCLUDE submakefile
will read in all the rules and macros from a file called submakefile and then continue reading the current Makefile. This can be done from the builtin Makefiles as well as any user Makefile. Nesting of includes is allowed; the maximum nesting level is indefinite.