diff --git a/jsonschema-validation.xml b/jsonschema-validation.xml
index 63475c2c..65754725 100644
--- a/jsonschema-validation.xml
+++ b/jsonschema-validation.xml
@@ -643,14 +643,14 @@
 
             <section title="Defined Formats">
 
-                <section title="Dates, Times, and Duration">
+                <section title="Dates, Times, Duration, and Period">
                     <t>
                         These attributes apply to string instances.
                     </t>
                     <t>
                         Date and time format names are derived from
                         <xref target="RFC3339">RFC 3339, section 5.6</xref>.
-                        The duration format is from the ISO 8601 ABNF as given
+                        The duration and period formats are from the ISO 8601 ABNF as given
                         in Appendix A of RFC 3339.
                     </t>
                     <t>
@@ -677,6 +677,11 @@
                                 a valid representation according to the "duration" ABNF rule
                                 (referenced above)
                             </t>
+                            <t hangText="period:">
+                                A string instance is valid against this attribute if it is
+                                a valid representation according to the "period" ABNF rule
+                                (referenced above)
+                            </t>
                         </list>
                     </t>
                     <t>