@@ -1935,13 +1935,17 @@ def non_math(self, s, loc, toks):
     float_literal = staticmethod(pyparsing_common.convertToFloat)
 
     def _make_space(self, percentage):
-        # All spaces are relative to em width
+        # In TeX, an em (the unit usually used to measure horizontal lengths)
+        # is not the width of the character 'm'; it is the same in different
+        # font styles (e.g. roman or italic). Mathtext, however, uses 'm' in
+        # the italic style so that horizontal spaces don't depend on the
+        # current font style.
         state = self.get_state()
         key = (state.font, state.fontsize, state.dpi)
         width = self._em_width_cache.get(key)
         if width is None:
             metrics = state.font_output.get_metrics(
-                state.font, mpl.rcParams['mathtext.default'], 'm',
+                'it', mpl.rcParams['mathtext.default'], 'm',
                 state.fontsize, state.dpi)
             width = metrics.advance
             self._em_width_cache[key] = width