diff --git a/src/position.cpp b/src/position.cpp
index b6dc31b4..42c1adff 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -272,7 +272,7 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
   // 5-6. Halfmove clock and fullmove number
   ss >> std::skipws >> st->rule50 >> gamePly;
 
-  // Convert from fullmove starting from 1 to ply starting from 0,
+  // Convert from fullmove starting from 1 to gamePly starting from 0,
   // handle also common incorrect FEN with fullmove = 0.
   gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
 
@@ -1071,11 +1071,10 @@ bool Position::is_draw(int ply) const {
   {
       stp = stp->previous->previous;
 
-      // At root position ply is 1, so return a draw score if a position
-      // repeats once earlier but strictly after the root, or repeats twice
-      // before or at the root.
+      // Return a draw score if a position repeats once earlier but strictly
+      // after the root, or repeats twice before or at the root.
       if (   stp->key == st->key
-          && ++cnt + (ply - 1 > i) == 2)
+          && ++cnt + (ply > i) == 2)
           return true;
   }
 
diff --git a/src/search.cpp b/src/search.cpp
index b68c8b78..eb8aea24 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -324,7 +324,7 @@ void MainThread::search() {
 
 void Thread::search() {
 
-  Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
+  Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
   Value bestValue, alpha, beta, delta;
   Move easyMove = MOVE_NONE;
   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
@@ -527,7 +527,7 @@ namespace {
   Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
 
     const bool PvNode = NT == PV;
-    const bool rootNode = PvNode && (ss-1)->ply == 0;
+    const bool rootNode = PvNode && ss->ply == 0;
 
     assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
     assert(PvNode || (alpha == beta - 1));
@@ -553,15 +553,14 @@ namespace {
     moveCount = quietCount = ss->moveCount = 0;
     ss->statScore = 0;
     bestValue = -VALUE_INFINITE;
-    ss->ply = (ss-1)->ply + 1;
 
     // Check for the available remaining time
     if (thisThread == Threads.main())
         static_cast<MainThread*>(thisThread)->check_time();
 
-    // Used to send selDepth info to GUI
-    if (PvNode && thisThread->selDepth < ss->ply)
-        thisThread->selDepth = ss->ply;
+    // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
+    if (PvNode && thisThread->selDepth < ss->ply + 1)
+        thisThread->selDepth = ss->ply + 1;
 
     if (!rootNode)
     {
@@ -584,6 +583,7 @@ namespace {
 
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
+    (ss+1)->ply = ss->ply + 1;
     ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
     ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
     (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
@@ -649,8 +649,8 @@ namespace {
 
                 int drawScore = TB::UseRule50 ? 1 : 0;
 
-                value =  v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
-                       : v >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply
+                value =  v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
+                       : v >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply - 1
                                         :  VALUE_DRAW + 2 * v * drawScore;
 
                 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
@@ -1164,7 +1164,7 @@ moves_loop: // When in check search starts from here
     }
 
     ss->currentMove = bestMove = MOVE_NONE;
-    ss->ply = (ss-1)->ply + 1;
+    (ss+1)->ply = ss->ply + 1;
     moveCount = 0;
 
     // Check for an instant draw or if the maximum ply has been reached
@@ -1179,7 +1179,6 @@ moves_loop: // When in check search starts from here
     // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
     ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
                                                   : DEPTH_QS_NO_CHECKS;
-
     // Transposition table lookup
     posKey = pos.key();
     tte = TT.probe(posKey, ttHit);