diff --git a/doc/rst/source/explain_core.rst_ b/doc/rst/source/explain_core.rst_
index 6c58a331214..b427144f76e 100644
--- a/doc/rst/source/explain_core.rst_
+++ b/doc/rst/source/explain_core.rst_
@@ -1,3 +1,4 @@
 **-x**\ [[-]\ *n*] :ref:`(more ...) <core_full>`
-    Limit number of cores used in multi-threaded algorithms (OpenMP required for most of modules,
-    or Glib for grdfilter, gmt|grdgravmag3d).
+    Limit number of cores used in multi-threaded algorithms. Multi-threaded behavior is enabled by default.
+    That covers the modules that implement the OpenMP API (required at compiling stage) and GThreads (Glib)
+    for the grdfilter module.
diff --git a/src/gmt_init.c b/src/gmt_init.c
index a492306137a..207d3ed6d1b 100644
--- a/src/gmt_init.c
+++ b/src/gmt_init.c
@@ -2607,8 +2607,8 @@ GMT_LOCAL int gmtinit_parse_U_option (struct GMT_CTRL *GMT, char *item) {
 }
 
 /*! -x[[-]<ncores>] */
+GMT_LOCAL int gmtinit_parse_x_option (struct GMT_CTRL *GMT, char *arg) {	/* Only effective if MP is enabled */
 #ifdef GMT_MP_ENABLED
-GMT_LOCAL int gmtinit_parse_x_option (struct GMT_CTRL *GMT, char *arg) {
 	GMT->common.x.active = true;
 	if (!arg) return (GMT_PARSE_ERROR);	/* -x requires a non-NULL argument */
 	if (arg[0] == '\0')	/* Use all processors */
@@ -2622,9 +2622,9 @@ GMT_LOCAL int gmtinit_parse_x_option (struct GMT_CTRL *GMT, char *arg) {
 		GMT->common.x.n_threads = MAX(gmtlib_get_num_processors() - abs (GMT->common.x.n_threads), 1);		/* Max-n but at least one */
 	if (GMT->current.setting.max_cores)	/* Limit to max core defaults setting */
 		GMT->common.x.n_threads = GMT->current.setting.max_cores;
+#endif
 	return (GMT_NOERROR);
 }
-#endif
 
 /*! . */
 GMT_LOCAL int gmtinit_parse_colon_option (struct GMT_CTRL *GMT, char *item) {
@@ -9151,9 +9151,12 @@ int gmt_default_error (struct GMT_CTRL *GMT, char option) {
 		case 's': error += GMT->common.s.active == false; break;
 		case 't': error += GMT->common.t.active == false; break;
 		case 'w': error += GMT->common.w.active == false; break;
-#if defined(GMT_MP_ENABLED)
-		case 'x': error += GMT->common.x.active == false; break;
+		case 'x': error += GMT->common.x.active == false;
+#if !defined(GMT_MP_ENABLED)
+		error --;
+		GMT_Report (GMT->parent, GMT_MSG_INFORMATION, "Option -x: GMT is not compiled with parallel support. Only one core is used\n");
 #endif
+		break;
 		case ':': error += GMT->common.colon.active == false; break;
 
 		default:
@@ -18792,12 +18795,13 @@ int gmt_parse_common_options (struct GMT_CTRL *GMT, char *list, char option, cha
 			error += gmt_M_more_than_once (GMT, GMT->common.w.active) || gmtinit_parse_w_option (GMT, item);
 			break;
 
-#ifdef GMT_MP_ENABLED
 		case 'x':
 			error += (gmt_M_more_than_once (GMT, GMT->common.x.active) || gmtinit_parse_x_option (GMT, item));
 			GMT->common.x.active = true;
-			break;
+#if !defined(GMT_MP_ENABLED)
+			GMT_Report (GMT->parent, GMT_MSG_WARNING, "Option -x: GMT is not compiled with parallel support. Only one core is used\n");
 #endif
+			break;
 
 		case ':':
 			error += (gmt_M_more_than_once (GMT, GMT->common.colon.active) || gmtinit_parse_colon_option (GMT, item));
diff --git a/src/grdfilter.c b/src/grdfilter.c
index 7c67882b7e4..c70cd9b8cb6 100644
--- a/src/grdfilter.c
+++ b/src/grdfilter.c
@@ -896,9 +896,9 @@ static int parse (struct GMT_CTRL *GMT, struct GRDFILTER_CTRL *Ctrl, struct GMT_
 #ifdef HAVE_GLIB_GTHREAD
 	/* Make the default equal to the OMP case where we use all threads if not stated otherwise. */
 	if (Ctrl->F.custom && GMT->common.x.n_threads > 1) {
-		//GMT->common.x.n_threads = 1;
-		//GMT->common.x.active = false;
-		//GMT_Report (GMT->parent, GMT_MSG_WARNING, "Sorry, custom filtering does not support multiple threads. Reset to 1.\n" );
+		GMT->common.x.n_threads = 1;
+		GMT->common.x.active = false;
+		GMT_Report (GMT->parent, GMT_MSG_WARNING, "Sorry, custom filtering does not support multiple threads. Reset to 1.\n" );
 	}
 	if (!GMT->common.x.active) {
 		GMT->common.x.n_threads = API->n_cores;
diff --git a/src/potential/gmtgravmag3d.c b/src/potential/gmtgravmag3d.c
index ea14836a81f..1c2da2a8828 100644
--- a/src/potential/gmtgravmag3d.c
+++ b/src/potential/gmtgravmag3d.c
@@ -453,11 +453,6 @@ static int parse(struct GMT_CTRL *GMT, struct GMTGRAVMAG3D_CTRL *Ctrl, struct GM
 		}
 	}
 
-#ifdef HAVE_GLIB_GTHREAD
-	/* Make the default equal to the OMP case where we use all threads if not stated otherwise. */
-	if (!GMT->common.x.active) GMT->common.x.n_threads = API->n_cores;
-#endif
-
 	n_errors += gmt_M_check_condition(GMT, Ctrl->S.active && (Ctrl->S.radius <= 0.0 || gmt_M_is_dnan (Ctrl->S.radius)),
 	                                  "Option -S: Radius is NaN or negative\n");
 	n_errors += gmt_M_check_condition(GMT, !Ctrl->T.active && !Ctrl->M.active, "Options -T or -M are mandatory\n");
diff --git a/src/potential/grdgravmag3d.c b/src/potential/grdgravmag3d.c
index cbb40c6b925..168384fb56c 100644
--- a/src/potential/grdgravmag3d.c
+++ b/src/potential/grdgravmag3d.c
@@ -448,11 +448,6 @@ static int parse(struct GMT_CTRL *GMT, struct GRDGRAVMAG3D_CTRL *Ctrl, struct GM
 		}
 	}
 
-#ifdef HAVE_GLIB_GTHREAD
-	/* Make the default equal to the OMP case where we use all threads if not stated otherwise. */
-	if (!GMT->common.x.active) GMT->common.x.n_threads = API->n_cores;
-#endif
-
 	n_errors += gmt_M_check_condition(GMT, !Ctrl->In.file[0], "Must specify input file\n");
 	n_errors += gmt_M_check_condition(GMT, Ctrl->S.active && (Ctrl->S.radius <= 0.0 || gmt_M_is_dnan(Ctrl->S.radius)),
 	                                "Option -S: Radius is NaN or negative\n");