diff --git a/src/CMI.hpp b/src/CMI.hpp
index 283ffd0..ea4ee65 100644
--- a/src/CMI.hpp
+++ b/src/CMI.hpp
@@ -14,6 +14,7 @@ namespace CMI {
  * to the CMI::HalfMove remains in the move tree.
  */
 class HalfMove {
+protected:
   HalfMove *parent = nullptr;
   HalfMove *mainline = nullptr;
   std::vector<HalfMove *> variations;