diff --git a/principles/index.html b/principles/index.html
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--- a/principles/index.html
+++ b/principles/index.html
@@ -117,7 +117,7 @@ <h4>Users should be able to investigate how data about them is used and shared.<
 
       <p>Users should be able to learn what measurements they may participate in.</p>
       <p>Users should be able to learn what level of risk of re-identification or cross-context data-sharing is possible. 
-        <br><i>See also: comprehensibility.</i></p>
+        <br><i>See also: <a href="#comprehensibility">comprehensibility</a>.</i></p>
     </section>
     <section>
       <h4>Researchers, regulators and auditors should be able to investigate how a system is used and whether abuse is occurring.</h4>
@@ -133,6 +133,25 @@ <h4>Researchers, regulators and auditors should be able to investigate how a sys
     <section>
       <h4>When abuse happens, there must be a mechanism to identify the abuse, limit further access and enable consequences.</h4>
     </section>
+    <section id="comprehensibility">
+      <h3>Comprehensibility</h3>
+      <section>
+        <h4>Users should be able to understand the implications of their participation in advertising systems.</h4>
+        <p>Users cannot be expected to understand the details of the process by which their privacy is protected, the mathematical guarantees of differential privacy, or similar concepts. However, users should still be able to see the implications of participation, including:</p>
+          <ul>
+            <li>the capabilities and what different advertising participants may learn about them;</li> 
+            <li>the likely, and unlikely, risks, including identification, disclosure and manipulation; and,</li> 
+            <li>whom they are trusting to protect their privacy.</li>
+          </ul>
+        <p>Comprehensibility doesn't require that every user understands or has a correct mental model. An interested user should be able to learn enough to predict the implications such that they can make informed decisions about participation.</p>
+      </section>
+      <section>
+        <h4>Reviewers, researchers, and policymakers should be able to understand the implications of advertising systems on users, websites, and advertisers.</h4>
+        <p>Unlike even the interested user, reviewers should be able to understand the full spectrum of potential implications for different users and for the ecosystem as a whole. This requires explainability of the implications to non-expert but professionally engaged interested observers.
+          <br><i>See also: <a href="#accountability">accountability</a>.</i></p>
+        </p>
+      </section>
+    </section>
   </section>
 
   <section class="appendix">