diff --git a/scripts/common.make b/scripts/common.make
index a2d1449..6d2fbc3 100644
--- a/scripts/common.make
+++ b/scripts/common.make
@@ -66,6 +66,10 @@
   SHARED_SUFFIX=
   ifndef THREADING
     # Auto-detect configurations with pthread.
+    # If the call to remove_unset_options returns "control", the symbols
+    # are confirmed set and we link with pthread.
+    # If the auto-detection fails, the result of the call is empty and
+    # we keep THREADING undefined.
     ifeq (control,$(call remove_unset_options,control MBEDTLS_THREADING_C MBEDTLS_THREADING_PTHREAD))
       THREADING := pthread
     endif