diff --git a/Makefile b/Makefile
index 945797181..fef643e4f 100644
--- a/Makefile
+++ b/Makefile
@@ -5,6 +5,8 @@ PREFIX=mbedtls_
 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
diff --git a/library/Makefile b/library/Makefile
index ce9c1eb99..e0141bfbe 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -5,6 +5,8 @@ CFLAGS	?= -O2
 WARNING_CFLAGS ?=  -Wall -W -Wdeclaration-after-statement
 LDFLAGS ?=
 
+# 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) )" ""
     # Set include dirs for Mbed TLS test environment