diff --git a/src/position.cpp b/src/position.cpp
index 40ea30f79f5..be9e7ac847f 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -770,9 +770,9 @@ void Position::do_move(Move m, StateInfo& newSt, const CheckInfo& ci, bool moveI
   Key k = st->key;
 
   // Copy some fields of old state to our new StateInfo object except the ones
-  // which are recalculated from scratch anyway, then switch our state pointer
-  // to point to the new, ready to be updated, state.
-  memcpy(&newSt, st, sizeof(ReducedStateInfo));
+  // which are going to be recalculated from scratch anyway, then switch our state
+  // pointer to point to the new, ready to be updated, state.
+  memcpy(&newSt, st, StateCopySize64 * sizeof(uint64_t));
 
   newSt.previous = st;
   st = &newSt;
diff --git a/src/position.h b/src/position.h
index 3014b2354b9..ff75848ecac 100644
--- a/src/position.h
+++ b/src/position.h
@@ -21,6 +21,7 @@
 #define POSITION_H_INCLUDED
 
 #include <cassert>
+#include <cstddef>
 
 #include "bitboard.h"
 #include "types.h"
@@ -44,7 +45,7 @@ struct CheckInfo {
 
 /// The StateInfo struct stores information we need to restore a Position
 /// object to its previous state when we retract a move. Whenever a move
-/// is made on the board (by calling Position::do_move), an StateInfo object
+/// is made on the board (by calling Position::do_move), a StateInfo object
 /// must be passed as a parameter.
 
 struct StateInfo {
@@ -60,13 +61,10 @@ struct StateInfo {
   StateInfo* previous;
 };
 
-struct ReducedStateInfo {
-  Key pawnKey, materialKey;
-  Value npMaterial[COLOR_NB];
-  int castleRights, rule50, pliesFromNull;
-  Score psqScore;
-  Square epSquare;
-};
+
+/// When making a move the current StateInfo up to 'key' excluded is copied to
+/// the new one. Here we calculate the quad words (64bits) needed to be copied.
+const size_t StateCopySize64 = offsetof(StateInfo, key) / sizeof(uint64_t) + 1;
 
 
 /// The position data structure. A position consists of the following data: