diff --git a/include/fwrite_le.h b/include/fwrite_le.h
index 58d6ef6..40a90f5 100644
--- a/include/fwrite_le.h
+++ b/include/fwrite_le.h
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0 OR Unlicense
+
 #ifndef _FWRITE_LE_H
 #define _FWRITE_LE_H
 
diff --git a/src/fwrite_le.c b/src/fwrite_le.c
index 7e0eb43..9919a25 100644
--- a/src/fwrite_le.c
+++ b/src/fwrite_le.c
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0 OR Unlicense
+
 #include "fwrite_le.h"
 
 #define ORDER_NATIVE_U32 0x01234567
diff --git a/src/template.c b/src/template.c
index b7fc4fa..21bf901 100644
--- a/src/template.c
+++ b/src/template.c
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0 OR Unlicense
+
 #include <inttypes.h>
 #include <math.h>
 #include <stdbool.h>