diff --git a/include/array_view.h b/include/array_view.h
index 100b238..26a1641 100644
--- a/include/array_view.h
+++ b/include/array_view.h
@@ -16,8 +16,8 @@
 
 #pragma once
 
-#ifndef ARRAY_VIEW_H
-#define ARRAY_VIEW_H
+#ifndef GSL_ARRAY_VIEW_H
+#define GSL_ARRAY_VIEW_H
 
 #include <new>
 #include <stdexcept>
@@ -2290,4 +2290,4 @@ general_array_view_iterator<ArrayView> operator+(typename general_array_view_ite
 
 #pragma pop_macro("_NOEXCEPT")
 
-#endif // ARRAY_VIEW_H
+#endif // GSL_ARRAY_VIEW_H
diff --git a/include/fail_fast.h b/include/fail_fast.h
index ebb86dd..78a5102 100644
--- a/include/fail_fast.h
+++ b/include/fail_fast.h
@@ -16,8 +16,8 @@
 
 #pragma once
 
-#ifndef FAIL_FAST_H
-#define FAIL_FAST_H
+#ifndef GSL_FAIL_FAST_H
+#define GSL_FAIL_FAST_H
 
 #include <exception>
 #include <stdexcept>
@@ -49,4 +49,4 @@ inline void fail_fast_assert(bool cond, const char* const) { if (!cond) std::ter
 
 }
 
-#endif // FAIL_FAST_H
+#endif // GSL_FAIL_FAST_H
diff --git a/include/gsl.h b/include/gsl.h
index 4eeac6d..e920c70 100644
--- a/include/gsl.h
+++ b/include/gsl.h
@@ -16,8 +16,8 @@
 
 #pragma once
 
-#ifndef GSL_H
-#define GSL_H
+#ifndef GSL_GSL_H
+#define GSL_GSL_H
 
 #include "array_view.h"     // array_view, strided_array_view...
 #include "string_view.h"    // zstring, string_view, zstring_builder...
@@ -289,4 +289,4 @@ template<class T> using maybe_null = maybe_null_ret<T>;
 
 } // namespace Guide
 
-#endif // GSL_H
+#endif // GSL_GSL_H
diff --git a/include/string_view.h b/include/string_view.h
index d12fdc0..e683c7a 100644
--- a/include/string_view.h
+++ b/include/string_view.h
@@ -16,8 +16,8 @@
 
 #pragma once
 
-#ifndef STRING_VIEW_H
-#define STRING_VIEW_H
+#ifndef GSL_STRING_VIEW_H
+#define GSL_STRING_VIEW_H
 
 #include "array_view.h"
 #include <cstring>
@@ -180,4 +180,4 @@ template <size_t Max = dynamic_range>
 using wzstring_builder = basic_zstring_builder<wchar_t, Max>;
 }
 
-#endif // STRING_VIEW_H
+#endif // GSL_STRING_VIEW_H