diff --git a/src/include/ngspice/inertial.h b/src/include/ngspice/inertial.h
new file mode 100644
index 000000000..1fb526155
--- /dev/null
+++ b/src/include/ngspice/inertial.h
@@ -0,0 +1,22 @@
+/* Definitions for inertial transport for XSPICE digital models. */
+
+/* Determine whether inertial transport should be used. */
+
+/* Values for control variable, "digital_delay_type". */
+
+#define DEFAULT_TRANSPORT  0
+#define DEFAULT_INERTIAL   1
+#define OVERRIDE_TRANSPORT 2
+#define OVERRIDE_INERTIAL  3
+
+enum param_vals {Off, On, Not_set};
+Mif_Boolean_t cm_is_inertial(enum param_vals param);
+
+/* Extra state data for inertial delays, one structure per output. */
+
+struct idata {
+    double          when;
+    Digital_State_t prev;
+};
+
+