diff --git a/storage.bs b/storage.bs
index 9996a34..f92d6e9 100644
--- a/storage.bs
+++ b/storage.bs
@@ -207,10 +207,9 @@ anticipated that some APIs will be applicable to both <a>storage types</a> going
 <h3 id=storage-keys>Storage keys</h3>
 
 <p>A <dfn export>storage key</dfn> is a <a>tuple</a> consisting of an
-<dfn for="storage key">origin</dfn> (an <a for=/>origin</a>). [[!HTML]]
-
-<p class=XXX>This is expected to change; see
-<a href="https://privacycg.github.io/storage-partitioning/">Client-Side Storage Partitioning</a>.
+<dfn for="storage key">origin</dfn> (an <a for=/>origin</a>), a
+<dfn for="storage key">top-level site</dfn> (a <a for=/>site</a>), and a
+<dfn for="storage key">cross-site ancestry</dfn> (a boolean). [[!HTML]]
 
 <div algorithm>
 <p>To <dfn export>obtain a storage key</dfn>, given an <a for=/>environment</a>
@@ -239,7 +238,18 @@ anticipated that some APIs will be applicable to both <a>storage types</a> going
  <a>environment settings object</a>; otherwise <var>environment</var>'s
  <a for=environment>creation URL</a>'s <a for=url>origin</a>.
 
- <li><p>Return a <a>tuple</a> consisting of <var>origin</var>.
+ <li><p>Let <var>topLevelOrigin</var> be <var>environment</var>'s
+ <a for=environment>top-level origin</a>.
+
+ <li><p>If <var>topLevelOrigin</var> is null, then set it to <var>origin</var>.
+
+ <li><p>Let <var>topLevelSite</var> be the result of <a>obtaining a site</a> given
+ <var>topLevelOrigin</var>.
+
+ <li><p>Let <var>crossSiteAncestry</var> be <var>environment</var>'s
+ <a for=environment>cross-site ancestry</a>.
+
+ <li><p>Return (<var>origin</var>, <var>topLevelSite</var>, <var>crossSiteAncestry</var>).
 </ol>
 </div>
 
@@ -251,6 +261,13 @@ anticipated that some APIs will be applicable to both <a>storage types</a> going
  <li><p>If <var>A</var>'s <a for="storage key">origin</a> is not <a>same origin</a> with
  <var>B</var>'s <a for="storage key">origin</a>, then return false.
 
+ <li><p>If <var>A</var>'s <a for="storage key">top-level site</a> is not
+ <a for="site">same site</a> with <var>B</var>'s <a for="storage key">top-level site</a>,
+ then return false.
+
+ <li><p>If <var>A</var>'s <a for="storage key">cross-site ancestry</a> is not
+ <var>B</var>'s <a for="storage key">cross-site ancestry</a>, then return false.
+
  <li><p>Return true.
 </ol>
 </div>