diff --git a/include/boost/bind/placeholders.hpp b/include/boost/bind/placeholders.hpp
index bda94bc..da6397f 100644
--- a/include/boost/bind/placeholders.hpp
+++ b/include/boost/bind/placeholders.hpp
@@ -20,17 +20,23 @@
 
 #include <boost/bind/arg.hpp>
 
+// The placeholders have been made static for use in precompiled headers.
+// They are in an unnamed namespace because underscored identifiers are
+// illegal at file scope.
+
 namespace
 {
-    boost::arg<1> _1;
-    boost::arg<2> _2;
-    boost::arg<3> _3;
-    boost::arg<4> _4;
-    boost::arg<5> _5;
-    boost::arg<6> _6;
-    boost::arg<7> _7;
-    boost::arg<8> _8;
-    boost::arg<9> _9;
-}
+
+static boost::arg<1> _1;
+static boost::arg<2> _2;
+static boost::arg<3> _3;
+static boost::arg<4> _4;
+static boost::arg<5> _5;
+static boost::arg<6> _6;
+static boost::arg<7> _7;
+static boost::arg<8> _8;
+static boost::arg<9> _9;
+
+} // unnamed namespace
 
 #endif // #ifndef BOOST_BIND_PLACEHOLDERS_HPP_INCLUDED