diff --git a/index.bs b/index.bs
index 08becf433..60d6dbfc3 100644
--- a/index.bs
+++ b/index.bs
@@ -9995,15 +9995,18 @@ Dictionary {{PeriodicWaveOptions}} Members</h5>
     ::
         The {{PeriodicWaveOptions/imag}} parameter represents an array of
         <code>sine</code> terms. The first element (index 0) does not
-        exist in the Fourier series.  The second element
+        exist in the Fourier series: it is set to 0 when the {{PeriodicWave}}
+        is constructed.  The second element
         (index 1) represents the fundamental frequency.  The
         third represents the first overtone and so on.
 
     : <dfn>real</dfn>
     ::
         The {{PeriodicWaveOptions/real}} parameter represents an array of
-        <code>cosine</code> terms. The first element (index 0) is the
-        DC-offset of the periodic waveform. The second element
+        <code>cosine</code> terms. The first element (index 0) is
+        ignored: it is set to 0 when the {{PeriodicWave}} is constructed,
+        so a {{PeriodicWave}} cannot produce a DC-offset. A DC-offset can
+        be added by connecting a {{ConstantSourceNode}}. The second element
         (index 1) represents the fundamental frequency.  The
         third represents the first overtone and so on.
 </dl>