diff --git a/Makefile.pdlibbuilder b/Makefile.pdlibbuilder
index 32c9543..1f7a6e3 100644
--- a/Makefile.pdlibbuilder
+++ b/Makefile.pdlibbuilder
@@ -102,6 +102,8 @@ version = 0.6.0
 # Optional user variables for make command line or environment:
 #
 # - PLATFORM
+# - extension
+# - floatsize
 #
 # Deprecated path variables:
 #
@@ -205,6 +207,19 @@ version = 0.6.0
 # will then be autodefined accordingly. In most cases no other variables need to
 # be overridden.
 #
+# extension:
+# Extension for the external to use. Example: m_amd64
+# A sane default is picked, but it is useful if you want to provide
+# co-installable externals for multiple platforms (for the same operating
+# systems)
+#
+# floatsize:
+# the size of the t_float in bits. Example: 32
+# t_float are usually single precision (32bit), which is the default.
+# For double precision use floatsize=64
+# When building double precision externals, you will want to set the extension
+# as well, e.g. extension=windows-amd64-64.dll (<system>-<cpu>-<floatsize>.<ext>)
+#
 # CPPFLAGS:
 # Preprocessor flags which are not strictly required for building.
 #