diff --git a/Makefile b/Makefile
index 1f0a673..6429762 100644
--- a/Makefile
+++ b/Makefile
@@ -124,13 +124,14 @@ else # ifeq ($(build),dev)
         #        fen.c
         #CPPFLAGS  += -DDEBUG_FEN                           # FEN decoding
         #        hash / TT
-        #CPPFLAGS  += -DZOBRIST_VERIFY                      # double chk zobrist
+        #CPPFLAGS  += -DZOBRIST_VERIFY                      # incr. zobrist check
         #CPPFLAGS  += -DPERFT_MOVE_HISTORY                  # perft, keep prev moves
         #        attack.c
         #CPPFLAGS  += -DDEBUG_ATTACK_ATTACKERS              # sq_attackers
         #CPPFLAGS  += -DDEBUG_ATTACK_PINNERS                # sq_pinners details
         #        eval.c eval-simple.c
-        CPPFLAGS  += -DDEBUG_EVAL                           # eval
+        CPPFLAGS  += -DDEBUG_EVAL                          # eval
+        CPPFLAGS  += -DPHASE_VERIFY                        # incr. phase check
 
         #        old unused flags
         #CPPFLAGS  += -DDEBUG_POS                           # position.c
diff --git a/src/eval-defs.c b/src/eval-defs.c
index 62332ac..8055a2a 100644
--- a/src/eval-defs.c
+++ b/src/eval-defs.c
@@ -22,6 +22,10 @@
 //#include "eval-simple.h"
 //#include "eval.h"
 
+phase_t piece_phase[PIECE_TYPE_NB] = {
+    0, P_PHASE, N_PHASE, B_PHASE, R_PHASE, Q_PHASE, 0
+};
+
 /* eval parameters definition. */
 static const struct ev_params ev_param_def [EV_PARAMS_NB] = {
     /*                type      setable def   min    max  name */
diff --git a/src/eval-defs.h b/src/eval-defs.h
index bd3881c..18ed3b5 100644
--- a/src/eval-defs.h
+++ b/src/eval-defs.h
@@ -36,18 +36,24 @@ enum {
     Q_PHASE = 4,
     ALL_PHASE = P_PHASE*16 + N_PHASE*4 + B_PHASE*4 + R_PHASE*4 + Q_PHASE*2
 };
+extern phase_t piece_phase[PIECE_TYPE_NB];
+
+static inline phase_t pt_phase(piece_type_t pt)
+{
+    return piece_phase[pt];
+}
 
 /* max pieces eval is 9*QUEEN_VALUE + 2*ROOK_VALUE + 2*BISHOP_VALUE
  * + 2*KNIGHT_VALUE which is (for a pawn valued at 100) well less than 15,000.
  */
 #define EVAL_MAX     (SHRT_MAX)                   /* 32767 */
 #define EVAL_MIN     (-EVAL_MAX)
-
+#define EVAL_DRAW    0
 #define EVAL_INV     EVAL_MIN
 
 #define EVAL_MATE    30000
 
-/* eval parameters */
+/* engine parameters */
 enum {
     WT_MAT,
     WT_PST,
diff --git a/src/eval.c b/src/eval.c
index 9f2fb64..1d09a3f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -19,6 +19,50 @@
 #include "position.h"
 #include "eval.h"
 #include "eval-simple.h"
+#include "hist.h"
+
+/**
+ * calc_phase - calculate position phase
+ * @pos: &position
+ *
+ * This function should be calculated when a new position is setup, or as
+ * a verification of an incremental one.
+ * Note: phase is *not* clamped, to avoid update errors.
+ *
+ * @return: phase value
+ */
+phase_t calc_phase(pos_t *pos)
+{
+    int phase = ALL_PHASE;
+    for (piece_type_t pt = PAWN; pt < KING; ++pt)
+        phase -= piece_phase[pt] * popcount64(pos->bb[WHITE][pt] | pos->bb[BLACK][pt]);
+#   ifdef DEBUG_PHASE
+    printf("calc phase:%d\n", phase);
+#   endif
+    return phase;
+}
+
+/**
+ * phase_verify() - verify position phase.
+ * @pos: &position
+ *
+ * Verify that position phase is correct (a full recalculation is performed).
+ * This function is inactive if PHASE_VERIFY is not set.
+ *
+ * @return: True if phase key is OK, no return otherwise.
+ */
+#ifdef PHASE_VERIFY
+bool phase_verify(pos_t *pos)
+{
+    phase_t verif = calc_phase(pos);
+    if (pos->phase != verif) {
+        warn(true, "warn phase=%d verif=%d\n", pos->phase, verif);
+        hist_print(pos);
+        bug_on(pos->phase != verif);
+    }
+    return true;
+}
+#endif
 
 /**
  * calc_phase - calculate position phase
@@ -30,7 +74,7 @@
  *
  * @return:
  */
-phase_t calc_phase(pos_t *pos)
+__unused static phase_t calc_phase2(pos_t *pos)
 {
     int phase = ALL_PHASE;
     phase -= P_PHASE * popcount64(pos->bb[WHITE][PAWN]   | pos->bb[BLACK][PAWN]);
@@ -41,7 +85,7 @@ phase_t calc_phase(pos_t *pos)
 
     phase = max(phase, 0);
 #   ifdef DEBUG_EVAL
-    printf("calc phase:%d\n", phase);
+    printf("calc phase:%d verif=%d\n", phase, calc_phase(pos));
 #   endif
     return phase;
 }
diff --git a/src/eval.h b/src/eval.h
index 2c2185a..ded6422 100644
--- a/src/eval.h
+++ b/src/eval.h
@@ -20,6 +20,11 @@
 #include "eval-defs.h"
 
 phase_t calc_phase(pos_t *pos);
+#ifdef PHASE_VERIFY
+bool phase_verify(pos_t *pos);
+#else
+#define phase_verify(p) true
+#endif
 
 eval_t eval_mobility(pos_t *pos, bool color);
 eval_t eval_square_control(pos_t *pos, bool color);
diff --git a/src/hash.c b/src/hash.c
index 989005a..15c489d 100644
--- a/src/hash.c
+++ b/src/hash.c
@@ -97,12 +97,11 @@ hkey_t zobrist_calc(pos_t *pos)
  * @pos: &position
  *
  * Verify that position Zobrist key matches a full Zobrist calculation.
- * This function cannot be called if ZOBRIST_VERIFY is not set.
+ * This function is inactive if ZOBRIST_VERIFY is not set.
  *
- * @return: True if Zobrist key is OK.
+ * @return: True if Zobrist key is OK, no return otherwise.
  */
 #ifdef ZOBRIST_VERIFY
-
 bool zobrist_verify(pos_t *pos)
 {
     hkey_t diff, key = zobrist_calc(pos);
diff --git a/src/move-do.c b/src/move-do.c
index c64cd5a..819afa3 100644
--- a/src/move-do.c
+++ b/src/move-do.c
@@ -79,16 +79,21 @@ pos_t *move_do(pos_t *pos, const move_t move, state_t *state)
 
     if (is_promotion(move)) {
         bug_on(sq_rank(to) != sq_rel_rank(RANK_8, us));
-        new_piece = MAKE_PIECE(move_promoted(move), us);
+        piece_t promoted = move_promoted(move);
+        new_piece = MAKE_PIECE(promoted, us);
+        pos->phase += pt_phase(PAWN) - pt_phase(promoted);
     }
 
     if (captured != EMPTY) {
         pos->ply50 = 0;
-        bug_on(pos->board[to] == EMPTY || COLOR(pos->captured) != them);
+        pos->phase += pt_phase(PIECE(captured));
+
+        bug_on(COLOR(captured) != them);
         key ^= zobrist_pieces[captured][to];
         pos_clr_sq(pos, to);                      /* clear square */
     } else if (is_castle(move)) {                 /* handle rook move */
         square_t rookfrom, rookto;
+
         if (to > from) {
             rookfrom = sq_rel(H1, us);
             rookto = sq_rel(F1, us);
@@ -105,6 +110,7 @@ pos_t *move_do(pos_t *pos, const move_t move, state_t *state)
         pos->ply50 = 0;
         if (from + up + up == to) {               /* if pawn double push, set e.p. */
             square_t ep = from + up;
+
             if (bb_pawn_attacks[us][ep] & pos->bb[them][PAWN]) {
                 pos->en_passant = ep;
                 key ^= zobrist_ep[EP_ZOBRIST_IDX(pos->en_passant)];
@@ -112,6 +118,8 @@ pos_t *move_do(pos_t *pos, const move_t move, state_t *state)
         } else if (is_enpassant(move)) {          /* clear grabbed pawn */
             square_t grabbed = to - up;
             piece_t pc = pos->board[grabbed];
+
+            pos->phase += pt_phase(PAWN);
             key ^= zobrist_pieces[pc][grabbed];
             pos_clr_sq(pos, grabbed);
         }
@@ -155,7 +163,12 @@ pos_t *move_do(pos_t *pos, const move_t move, state_t *state)
     pos->key = key;
 
     pos->repcount = pos_repcount(pos);
+
+    /* consistency checks.
+     * Checks done only if compiled with '-DZOBRIST_VERIFY' or '-DPHASE_VERIFY'
+     */
     zobrist_verify(pos);
+    phase_verify(pos);
 
     return pos;
 }
diff --git a/src/position.c b/src/position.c
index 40da984..deb09a2 100644
--- a/src/position.c
+++ b/src/position.c
@@ -185,8 +185,8 @@ bitboard_t pos_checkers(const pos_t *pos, const color_t color)
  * pos_repcount() - return position repetition count.
  * @pos: &position to search
  *
- * Attention: positions before (and including) root position repcount is
- * decreased by one. See do_moves() in uci.c.
+ * Attention: positions before (and including) root position repcount are
+ * already decreased by one. See do_moves() in uci.c.
  *
  * @return: The number of repetitions in history, zero otherwise.
  */