diff --git a/nix.conf.example b/nix.conf.example
index 97c6f4b15c..13b4cfa3d9 100644
--- a/nix.conf.example
+++ b/nix.conf.example
@@ -97,3 +97,23 @@
 # Example:
 #   build-users = nix-builder-1 nix-builder-2 nix-builder-3
 #build-users =
+
+
+### Option `system'
+#
+# This option specifies the canonical Nix system name of the current
+# installation, such as `i686-linux' or `powerpc-darwin'.  Nix can
+# only build derivations whose `system' attribute equals the value
+# specified here.  In general, it never makes sense to modify this
+# value from its default, since you can use it to `lie' about the
+# platform you are building on (e.g., perform a Mac OS build on a
+# Linux machine; the result would obviously be wrong).  It only makes
+# sense if the Nix binaries can run on multiple platforms, e.g.,
+# `universal binaries' that run on `powerpc-darwin' and `i686-darwin'.
+#
+# It defaults to the canonical Nix system name detected by `configure'
+# at build time.
+#
+# Example:
+#   system = i686-darwin
+#system =
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 97528f123e..fb24e83298 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -93,6 +93,8 @@ static void initAndRun(int argc, char * * argv)
     nixDBPath = getEnv("NIX_DB_DIR", nixStateDir + "/db");
     nixConfDir = canonPath(getEnv("NIX_CONF_DIR", NIX_CONF_DIR));
 
+    thisSystem = querySetting("system", SYSTEM);
+
     /* Catch SIGINT. */
     struct sigaction act, oact;
     act.sa_handler = sigintHandler;
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 194e1165fa..ac8e19b7d4 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -17,6 +17,7 @@ bool tryFallback = false;
 Verbosity buildVerbosity = lvlInfo;
 unsigned int maxBuildJobs = 1;
 bool readOnlyMode = false;
+string thisSystem = "unset";
 
 
 static bool settingsRead = false;
@@ -26,7 +27,7 @@ static map<string, Strings> settings;
 
 string & at(Strings & ss, unsigned int n)
 {
-    Strings::iterator i =ss.begin();
+    Strings::iterator i = ss.begin();
     advance(i, n);
     return *i;
 }
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index b5de709f7b..408076b751 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -53,6 +53,9 @@ extern unsigned int maxBuildJobs;
    database. */
 extern bool readOnlyMode;
 
+/* The canonical system name, as returned by config.guess. */ 
+extern string thisSystem;
+
 
 Strings querySetting(const string & name, const Strings & def);
 
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 973443cf13..9e3e0bae20 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -16,9 +16,6 @@
 #include "util.hh"
 
 
-string thisSystem = SYSTEM;
-
-
 Error::Error(const format & f)
 {
     err = f.str();
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 05cf777f1e..92bdf50d31 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -55,10 +55,6 @@ typedef list<Path> Paths;
 typedef set<Path> PathSet;
 
 
-/* The canonical system name, as returned by config.guess. */ 
-extern string thisSystem;
-
-
 /* Return an environment variable. */
 string getEnv(const string & key, const string & def = "");