diff --git a/graf3d/x3d/src/x3d.c b/builtins/x3d/x3d.c
similarity index 100%
rename from graf3d/x3d/src/x3d.c
rename to builtins/x3d/x3d.c
diff --git a/graf3d/x3d/src/x3d.h b/builtins/x3d/x3d.h
similarity index 100%
rename from graf3d/x3d/src/x3d.h
rename to builtins/x3d/x3d.h
diff --git a/documentation/doxygen/Doxyfile b/documentation/doxygen/Doxyfile
index c55705908c484..085ead947bf48 100644
--- a/documentation/doxygen/Doxyfile
+++ b/documentation/doxygen/Doxyfile
@@ -1087,7 +1087,6 @@ EXCLUDE                = ../../interpreter/ \
     ../../core/newdelete/ \
     ../../core/textinput/ \
     ../../builtins/ \
-    ../../graf3d/x3d/ \
     ../../documentation/doxygen/html
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
diff --git a/graf3d/x3d/CMakeLists.txt b/graf3d/x3d/CMakeLists.txt
index 223eab9bdadd1..f48ee049ffe96 100644
--- a/graf3d/x3d/CMakeLists.txt
+++ b/graf3d/x3d/CMakeLists.txt
@@ -13,9 +13,9 @@ ROOT_STANDARD_LIBRARY_PACKAGE(X3d
     TViewerX3D.h
     TX3DFrame.h
   SOURCES
-    src/x3d.c
     src/TViewerX3D.cxx
     src/TX3DFrame.cxx
+    ../../builtins/x3d/x3d.c
   LIBRARIES
     ${X11_LIBRARIES}
   DEPENDENCIES
@@ -23,4 +23,9 @@ ROOT_STANDARD_LIBRARY_PACKAGE(X3d
     Gui
 )
 
-target_include_directories(X3d PRIVATE ${X11_INCLUDE_DIR})
+# This ROOT package depends on a hard-coded builtin C library (x3d) that was forked in 1993 and slightly adapted for ROOT. See
+# https://github.com/root-project/root/blob/852600061bcacd9b255d44f6312c96b6b1e00a2d/x3d/inc/x3d.h
+# https://github.com/root-project/root/blob/852600061bcacd9b255d44f6312c96b6b1e00a2d/x3d/src/x3d.c
+# vs http://hpux.connect.org.uk/hppd/hpux/X11/Viewers/x3d-2.2/ download source and rename from .tar.gz to just .tar
+
+target_include_directories(X3d PRIVATE ${X11_INCLUDE_DIR} src/ ../../builtins/x3d/) # src for src/X3DDefs.h, ../../builtins/x3d/ for x3d.h