diff --git a/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php b/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
--- a/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
@@ -525,5 +525,48 @@
       $text);
   }
 
+  /**
+   * Build the prefixes for line IDs used to track inline comments.
+   *
+   * @return pair<wild, wild> Left and right prefixes.
+   */
+  protected function getLineIDPrefixes() {
+    // These look like "C123NL45", which means the line is line 45 on the
+    // "new" side of the file in changeset 123.
+
+    // The "C" stands for "changeset", and is followed by a changeset ID.
+
+    // "N" stands for "new" and means the comment should attach to the new file
+    // when stored. "O" stands for "old" and means the comment should attach to
+    // the old file. These are important because either the old or new part
+    // of a file may appear on the left or right side of the diff in the
+    // diff-of-diffs view.
+
+    // The "L" stands for "line" and is followed by the line number.
+
+    if ($this->getOldChangesetID()) {
+      $left_prefix = array();
+      $left_prefix[] = 'C';
+      $left_prefix[] = $this->getOldChangesetID();
+      $left_prefix[] = $this->getOldAttachesToNewFile() ? 'N' : 'O';
+      $left_prefix[] = 'L';
+      $left_prefix = implode('', $left_prefix);
+    } else {
+      $left_prefix = null;
+    }
+
+    if ($this->getNewChangesetID()) {
+      $right_prefix = array();
+      $right_prefix[] = 'C';
+      $right_prefix[] = $this->getNewChangesetID();
+      $right_prefix[] = $this->getNewAttachesToNewFile() ? 'N' : 'O';
+      $right_prefix[] = 'L';
+      $right_prefix = implode('', $right_prefix);
+    } else {
+      $right_prefix = null;
+    }
+
+    return array($left_prefix, $right_prefix);
+  }
 
 }
diff --git a/src/applications/differential/render/DifferentialChangesetOneUpRenderer.php b/src/applications/differential/render/DifferentialChangesetOneUpRenderer.php
--- a/src/applications/differential/render/DifferentialChangesetOneUpRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetOneUpRenderer.php
@@ -30,6 +30,8 @@
 
     $primitives = $this->buildPrimitives($range_start, $range_len);
 
+    list($left_prefix, $right_prefix) = $this->getLineIDPrefixes();
+
     $out = array();
     foreach ($primitives as $p) {
       $type = $p['type'];
@@ -43,18 +45,37 @@
             } else {
               $class = 'left';
             }
-            $out[] = phutil_tag('th', array(), $p['line']);
+
+            if ($left_prefix) {
+              $left_id = $left_prefix.$p['line'];
+            } else {
+              $left_id = null;
+            }
+            $out[] = phutil_tag('th', array('id' => $left_id), $p['line']);
+
             $out[] = phutil_tag('th', array());
             $out[] = phutil_tag('td', array('class' => $class), $p['render']);
-          } else if ($type == 'new') {
+          } else {
             if ($p['htype']) {
               $class = 'right new';
               $out[] = phutil_tag('th', array());
             } else {
               $class = 'right';
-              $out[] = phutil_tag('th', array(), $p['oline']);
+              if ($left_prefix) {
+                $left_id = $left_prefix.$p['oline'];
+              } else {
+                $left_id = null;
+              }
+              $out[] = phutil_tag('th', array('id' => $left_id), $p['oline']);
             }
-            $out[] = phutil_tag('th', array(), $p['line']);
+
+            if ($right_prefix) {
+              $right_id = $right_prefix.$p['line'];
+            } else {
+              $right_id = null;
+            }
+            $out[] = phutil_tag('th', array('id' => $right_id), $p['line']);
+
             $out[] = phutil_tag('td', array('class' => $class), $p['render']);
           }
           $out[] = hsprintf('</tr>');
diff --git a/src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php b/src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php
--- a/src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php
@@ -55,19 +55,8 @@
     $new_lines = $this->getNewLines();
     $gaps = $this->getGaps();
     $reference = $this->getRenderingReference();
-    $left_id = $this->getOldChangesetID();
-    $right_id = $this->getNewChangesetID();
 
-    // "N" stands for 'new' and means the comment should attach to the new file
-    // when stored, i.e. DifferentialInlineComment->setIsNewFile().
-    // "O" stands for 'old' and means the comment should attach to the old file.
-
-    $left_char = $this->getOldAttachesToNewFile()
-      ? 'N'
-      : 'O';
-    $right_char = $this->getNewAttachesToNewFile()
-      ? 'N'
-      : 'O';
+    list($left_prefix, $right_prefix) = $this->getLineIDPrefixes();
 
     $changeset = $this->getChangeset();
     $copy_lines = idx($changeset->getMetadata(), 'copy:lines', array());
@@ -234,14 +223,14 @@
         $html[] = $context_not_available;
       }
 
-      if ($o_num && $left_id) {
-        $o_id = 'C'.$left_id.$left_char.'L'.$o_num;
+      if ($o_num && $left_prefix) {
+        $o_id = $left_prefix.$o_num;
       } else {
         $o_id = null;
       }
 
-      if ($n_num && $right_id) {
-        $n_id = 'C'.$right_id.$right_char.'L'.$n_num;
+      if ($n_num && $right_prefix) {
+        $n_id = $right_prefix.$n_num;
       } else {
         $n_id = null;
       }
@@ -251,9 +240,6 @@
       // clipboard. See the 'phabricator-oncopy' behavior.
       $zero_space = "\xE2\x80\x8B";
 
-      // NOTE: The Javascript is sensitive to whitespace changes in this
-      // block!
-
       $html[] = phutil_tag('tr', array(), array(
         phutil_tag('th', array('id' => $o_id), $o_num),
         phutil_tag('td', array('class' => $o_classes), $o_text),