diff --git a/Makefile b/Makefile
index 9457971..fef643e 100644
--- a/Makefile
+++ b/Makefile
@@ -5,6 +5,8 @@
 PROGRAMS_DIR=./programs
 TESTS_DIR=./tests
 
+# Check test environment. If ../library is available then Mbed TLS is used.
+# Otherwise Mbed OS environment is used.
 DIR_FOR_MBED_TLS_ENV=./library
 ifneq "$(wildcard $(DIR_FOR_MBED_TLS_ENV) )" ""
 	LIBRARY_DIR=./library