diff --git a/src/c/alloc_2.d b/src/c/alloc_2.d
index fda99bd90..dbeb1ead1 100644
--- a/src/c/alloc_2.d
+++ b/src/c/alloc_2.d
@@ -443,7 +443,7 @@ cl_object_mark_proc(void *addr, struct GC_ms_entry *msp, struct GC_ms_entry *msl
     break;
   case t_instance:
     MAYBE_MARK(o->instance.slots);
-    MAYBE_MARK(o->instance.sig);
+    MAYBE_MARK(o->instance.slotds);
     MAYBE_MARK(o->instance.clas);
     break;
 # ifdef ECL_THREADS
@@ -682,7 +682,7 @@ ecl_alloc_instance(cl_index slots)
   i->instance.slots = (cl_object *)ecl_alloc(sizeof(cl_object) * slots);
   i->instance.length = slots;
   i->instance.entry = FEnot_funcallable_vararg;
-  i->instance.sig = ECL_UNBOUND;
+  i->instance.slotds = ECL_UNBOUND;
   return i;
 }
 
@@ -1012,7 +1012,7 @@ init_alloc(void)
     to_bitmap(&o, &(o.cclosure.file_position));
   type_info[t_instance].descriptor =
     to_bitmap(&o, &(o.instance.clas)) |
-    to_bitmap(&o, &(o.instance.sig)) |
+    to_bitmap(&o, &(o.instance.slotds)) |
     to_bitmap(&o, &(o.instance.slots));
 # ifdef ECL_THREADS
   type_info[t_process].descriptor =
diff --git a/src/c/clos/gfun.d b/src/c/clos/gfun.d
index 00d76c551..211ce4020 100644
--- a/src/c/clos/gfun.d
+++ b/src/c/clos/gfun.d
@@ -59,7 +59,7 @@ reshape_instance(cl_object x, int delta)
   cl_fixnum size = x->instance.length + delta;
   cl_object aux = ecl_allocate_instance(ECL_CLASS_OF(x), size);
   /* Except for the different size, this must match si_copy_instance */
-  aux->instance.sig = x->instance.sig;
+  aux->instance.slotds = x->instance.slotds;
   aux->instance.stamp = x->instance.stamp;
   aux->instance.class_stamp = x->instance.class_stamp;
   memcpy(aux->instance.slots, x->instance.slots,
diff --git a/src/c/clos/instance.d b/src/c/clos/instance.d
index 914cc262e..e40dc81e1 100644
--- a/src/c/clos/instance.d
+++ b/src/c/clos/instance.d
@@ -68,16 +68,16 @@ si_instance_get_stamp(cl_object x)
 }
 
 cl_object
-si_instance_sig(cl_object x)
+si_instance_slotds(cl_object x)
 {
-  @(return x->instance.sig);
+  @(return x->instance.slotds);
 }
 
 cl_object
 si_instance_sig_set(cl_object x)
 {
   x->instance.stamp = ECL_CLASS_OF(x)->instance.class_stamp;
-  @(return (x->instance.sig = ECL_CLASS_SLOTS(ECL_CLASS_OF(x))));
+  @(return (x->instance.slotds = ECL_CLASS_SLOTS(ECL_CLASS_OF(x))));
 }
 
 cl_object
@@ -296,7 +296,7 @@ si_copy_instance(cl_object x)
     FEwrong_type_nth_arg(@[si::copy-instance], 1, x, @[ext::instance]);
   }
   y = ecl_allocate_instance(x->instance.clas, x->instance.length);
-  y->instance.sig = x->instance.sig;
+  y->instance.slotds = x->instance.slotds;
   y->instance.stamp = x->instance.stamp;
   y->instance.class_stamp = x->instance.class_stamp;
   memcpy(y->instance.slots, x->instance.slots,
diff --git a/src/c/structure.d b/src/c/structure.d
index e662c7030..092f984ef 100644
--- a/src/c/structure.d
+++ b/src/c/structure.d
@@ -49,7 +49,7 @@ si_structure_subtype_p(cl_object x, cl_object y)
   ECL_STRUCT_SLOTS(x) = NULL;     /* for GC sake */
   ECL_STRUCT_LENGTH(x) = --narg;
   ECL_STRUCT_SLOTS(x) = (cl_object *)ecl_alloc_align(sizeof(cl_object)*narg, sizeof(cl_object));
-  x->instance.sig = ECL_UNBOUND;
+  x->instance.slotds = ECL_UNBOUND;
   if (narg >= ECL_SLOTS_LIMIT)
     FEerror("Limit on structure size exceeded: ~S slots requested.",
             1, ecl_make_fixnum(narg));
diff --git a/src/c/symbols_list.h b/src/c/symbols_list.h
index f54a5bb39..2c762e7ca 100755
--- a/src/c/symbols_list.h
+++ b/src/c/symbols_list.h
@@ -1297,7 +1297,7 @@ cl_symbols[] = {
 {SYS_ "INSTANCE-OBSOLETE-P", SI_ORDINARY, si_instance_obsolete_p, 1, OBJNULL},
 {SYS_ "INSTANCE-NEW-STAMP", SI_ORDINARY, si_instance_new_stamp, 1, OBJNULL},
 {SYS_ "INSTANCE-GET-STAMP", SI_ORDINARY, si_instance_get_stamp, 1, OBJNULL},
-{SYS_ "INSTANCE-SIG", SI_ORDINARY, si_instance_sig, 1, OBJNULL},
+{SYS_ "INSTANCE-SLOTDS", SI_ORDINARY, si_instance_slotds, 1, OBJNULL},
 {SYS_ "INSTANCE-SIG-SET", SI_ORDINARY, si_instance_sig_set, 1, OBJNULL},
 {SYS_ "INSTANCE-CLASS", SI_ORDINARY, si_instance_class, 1, OBJNULL},
 {SYS_ "INSTANCE-CLASS-SET", SI_ORDINARY, si_instance_class_set, 2, OBJNULL},
diff --git a/src/c/symbols_list2.h b/src/c/symbols_list2.h
index 67f2a7452..a28cfbe8e 100644
--- a/src/c/symbols_list2.h
+++ b/src/c/symbols_list2.h
@@ -1297,7 +1297,7 @@ cl_symbols[] = {
 {SYS_ "INSTANCE-OBSOLETE-P","si_instance_obsolete_p",1},
 {SYS_ "INSTANCE-NEW-STAMP","si_instance_new_stamp",1},
 {SYS_ "INSTANCE-GET-STAMP","si_instance_get_stamp",1},
-{SYS_ "INSTANCE-SIG","si_instance_sig",1},
+{SYS_ "INSTANCE-SLOTDS","si_instance_slotds",1},
 {SYS_ "INSTANCE-SIG-SET","si_instance_sig_set",1},
 {SYS_ "INSTANCE-CLASS","si_instance_class",1},
 {SYS_ "INSTANCE-CLASS-SET","si_instance_class_set",2},
diff --git a/src/clos/change.lsp b/src/clos/change.lsp
index 81d34201f..c7ad26dec 100644
--- a/src/clos/change.lsp
+++ b/src/clos/change.lsp
@@ -35,8 +35,8 @@
 
 (defmethod update-instance-for-different-class
     ((old-data standard-object) (new-data standard-object) &rest initargs)
-  (let ((old-local-slotds (si::instance-sig old-data))
-        (new-local-slotds (remove :instance (si::instance-sig new-data)
+  (let ((old-local-slotds (si::instance-slotds old-data))
+        (new-local-slotds (remove :instance (si::instance-slotds new-data)
                                   :test-not #'eq :key #'slot-definition-allocation))
         added-slots)
     (setf added-slots (set-difference (mapcar #'slot-definition-name new-local-slotds)
@@ -83,9 +83,9 @@
 ;;;
 ;;; PART 2: UPDATING AN INSTANCE THAT BECAME OBSOLETE
 ;;;
-;;; Each instance has a hidden field (readable with SI::INSTANCE-SIG),
-;;; which contains the list of slots of its class which are needed to
-;;; update it when it is obsolete. Generally
+;;; Each instance has a field (readable with SI::INSTANCE-SLOTDS),
+;;; which contains the list of slot definitions from its class which
+;;; are needed to update it when it is obsolete. Generally
 ;;;
 ;;;     (SI::INSTANCE-OBSOLETE-P x)
 ;;;
@@ -147,7 +147,7 @@
 
 (defun update-instance (instance)
   (let* ((class (class-of instance))
-         (old-slotds (si::instance-sig instance))
+         (old-slotds (si::instance-slotds instance))
          (new-slotds (class-slots class))
          (old-instance (si::copy-instance instance))
          (discarded-slots '())
diff --git a/src/clos/fixup.lsp b/src/clos/fixup.lsp
index e29f3504a..a56c65384 100644
--- a/src/clos/fixup.lsp
+++ b/src/clos/fixup.lsp
@@ -87,9 +87,7 @@
                                       (t
                                        old-class))))
       (si::instance-sig-set gfun)
-      (register-method-with-specializers method)
-      )
-    ))
+      (register-method-with-specializers method))))
 
 
 ;;; ---------------------------------------------------------------------
diff --git a/src/cmp/proclamations.lsp b/src/cmp/proclamations.lsp
index 91f19a05f..7f9d607aa 100644
--- a/src/cmp/proclamations.lsp
+++ b/src/cmp/proclamations.lsp
@@ -1428,7 +1428,7 @@
 #+clos
 (proclamation si:instance-ref (t fixnum) t :reader)
 #+clos
-(proclamation si::instance-sig (standard-object) list :reader)
+(proclamation si::instance-slotds (standard-object) list :reader)
 #+clos
 (proclamation si:instance-set (t fixnum t) t)
 #+clos
diff --git a/src/cmp/sysfun.lsp b/src/cmp/sysfun.lsp
index c82881ef1..ab6e8c107 100644
--- a/src/cmp/sysfun.lsp
+++ b/src/cmp/sysfun.lsp
@@ -827,8 +827,8 @@
  "(#0)->instance.slots[#1]")
 
 #+clos
-(def-inline si::instance-sig :unsafe (standard-object) list
- "(#0)->instance.sig")
+(def-inline si::instance-slotds :unsafe (standard-object) list
+ "(#0)->instance.slotds")
 
 #+clos
 (def-inline si:instance-set :unsafe (t fixnum t) t
diff --git a/src/h/external.h b/src/h/external.h
index 1095eb621..e84e1d742 100755
--- a/src/h/external.h
+++ b/src/h/external.h
@@ -843,7 +843,7 @@ extern ECL_API cl_object si_sl_makunbound(cl_object x, cl_object index);
 extern ECL_API cl_object si_instance_obsolete_p(cl_object x);
 extern ECL_API cl_object si_instance_new_stamp(cl_object x);
 extern ECL_API cl_object si_instance_get_stamp(cl_object x);
-extern ECL_API cl_object si_instance_sig(cl_object x);
+extern ECL_API cl_object si_instance_slotds(cl_object x);
 extern ECL_API cl_object si_instance_sig_set(cl_object x);
 
 extern ECL_API cl_object ecl_allocate_instance(cl_object clas, cl_index size);
diff --git a/src/h/object.h b/src/h/object.h
index 4b92b4679..851bff4ef 100644
--- a/src/h/object.h
+++ b/src/h/object.h
@@ -1044,7 +1044,7 @@ struct ecl_instance {            /* -- instance header -- */
         cl_index length;         /*  instance length      */
         cl_object clas;          /*  instance class       */
         cl_objectfn entry;       /*  entry address        */
-        cl_object sig;           /*  instance class slots */
+        cl_object slotds;        /*  slot definitions     */
         cl_object *slots;        /*  instance slots       */
         cl_index stamp;          /*  instance stamp       */
         cl_index class_stamp;    /*  class stamp          */