HTML/CSS/Form Attributes/multiple
"multiple" sets whether "option" elements can be multiple selected
<HTML>
<BODY>
<body onUnload="document.forms[0].reset()">
<form name="form1" method="post" action="">
<select name="Chapters" multiple>
<option>Chapter 1</option>
<option>Chapter 2</option>
<option>Chapter 3</option>
<option>Chapter 4</option>
<option>Chapter 5</option>
<option>Chapter 6</option>
</select>
</form>
</body>
</BODY>
</HTML>