diff --git a/README.md b/README.md
index d8009c6..3b5d1a2 100644
--- a/README.md
+++ b/README.md
@@ -1555,7 +1555,7 @@ p.add_argument('<name>', type=<type>, nargs='?/*')                # Optional arg
 ```
 
 * **Use `'help=<str>'` to set argument description that will be displayed in help message.**
-* **Use `'default=<el>'` to set option's default value.**
+* **Use `'default=<el>'` to set option's or optional argument's default value.**
 * **Use `'type=FileType(<mode>)'` for files. Accepts 'encoding', but 'newline' is None.**
 
 
diff --git a/index.html b/index.html
index f4d17c0..2b5717e 100644
--- a/index.html
+++ b/index.html
@@ -1319,7 +1319,7 @@ p.add_argument(<span class="hljs-string">'&lt;name&gt;'</span>, type=&lt;type&gt
 
 <ul>
 <li><strong>Use <code class="python hljs"><span class="hljs-string">'help=&lt;str&gt;'</span></code> to set argument description that will be displayed in help message.</strong></li>
-<li><strong>Use <code class="python hljs"><span class="hljs-string">'default=&lt;el&gt;'</span></code> to set option's default value.</strong></li>
+<li><strong>Use <code class="python hljs"><span class="hljs-string">'default=&lt;el&gt;'</span></code> to set option's or optional argument's default value.</strong></li>
 <li><strong>Use <code class="python hljs"><span class="hljs-string">'type=FileType(&lt;mode&gt;)'</span></code> for files. Accepts 'encoding', but 'newline' is None.</strong></li>
 </ul>
 <div><h2 id="open"><a href="#open" name="open">#</a>Open</h2><p><strong>Opens the file and returns a corresponding file object.</strong></p><pre><code class="python language-python hljs">&lt;file&gt; = open(&lt;path&gt;, mode=<span class="hljs-string">'r'</span>, encoding=<span class="hljs-keyword">None</span>, newline=<span class="hljs-keyword">None</span>)