diff --git a/src/backend/backend.c b/src/backend/backend.c
index bf5379d3..9f674737 100644
--- a/src/backend/backend.c
+++ b/src/backend/backend.c
@@ -14,14 +14,6 @@ backend_info_t *backend_list[NUM_BKEND] = {
 #endif
 };
 
-bool default_is_win_transparent(void *backend_data, win *w, void *win_data) {
-	return w->mode != WMODE_SOLID;
-}
-
-bool default_is_frame_transparent(void *backend_data, win *w, void *win_data) {
-	return w->frame_opacity != 1;
-}
-
 region_t get_damage(session_t *ps) {
 	region_t region;
 	auto buffer_age_fn = backend_list[ps->o.backend]->buffer_age;
diff --git a/src/backend/backend_common.c b/src/backend/backend_common.c
index acdbcdca..96985b46 100644
--- a/src/backend/backend_common.c
+++ b/src/backend/backend_common.c
@@ -244,3 +244,11 @@ shadow_picture_err:
 
 	return false;
 }
+
+bool default_is_win_transparent(void *backend_data, win *w, void *win_data) {
+	return w->mode != WMODE_SOLID;
+}
+
+bool default_is_frame_transparent(void *backend_data, win *w, void *win_data) {
+	return w->frame_opacity != 1;
+}
diff --git a/src/backend/backend_common.h b/src/backend/backend_common.h
index 3b6d9581..72321abe 100644
--- a/src/backend/backend_common.h
+++ b/src/backend/backend_common.h
@@ -19,3 +19,11 @@ solid_picture(session_t *ps, bool argb, double a, double r, double g, double b);
 
 xcb_image_t *
 make_shadow(xcb_connection_t *c, const conv *kernel, double opacity, int width, int height);
+
+/// The default implementation of `is_win_transparent`, it simply looks at win::mode. So
+/// this is not suitable for backends that alter the content of windows
+bool default_is_win_transparent(void *, win *, void *);
+
+/// The default implementation of `is_frame_transparent`, it uses win::frame_opacity. Same
+/// caveat as `default_is_win_transparent` applies.
+bool default_is_frame_transparent(void *, win *, void *);