grace_option/3 can be called in the initialisation file .gracerc or directly inside the program. Although some options can be processed even without Grace running, grace_option/3 in general has to be called only after Grace has been initialised with grace_start/1 . This is the case for the .gracerc file, however inside the program the user must make sure that this is really true. A frequent error is to call grace_option/3 as a query inside a Prolog source file, which does not work because grace_start/1 has not yet been called.