chained dropdown
by oterox
HTML
<select id="C_Country" name="C_Country" aria-required="true" class="required">
<option value=" " selected="">-- Please Select --</option>
<option value="AF">Afghanistan</option>
<option value="AX">Aland Islands</option>
<option value="USA">United States</option>
<option value="DZ">Algeria</option>
<option value="CA">Canada</option>
<option value="AD">Andorra</option>
</select>
<p><label for="C_State_Prov">State or Province <span title="required">*</span></label></p>
<select id="C_State_Prov" name="C_State_Prov" aria-required="true" class="required">
<option value="" selected="">-- Please Select --</option>
<optgroup label="United States">
<option value="AK">Alaska</option>
<option value="AL">Alabama</option>
<option value="AR">Arkansas</option>
<option value="AZ">Arizona</option>
<option value="CA">California</option>
</optgroup>
<optgroup label="Canada">
<option value="AB">Alberta</option>
<option value="BC">British Columbia</option>
<option value="MB">Manitoba</option>
</optgroup>
<optgroup label="Andorra">
<option value="AB">Paracuellos</option>
<option value="BC">Madrid</option>
<option value="MB">Alcobendas</option>
</optgroup>
</select>
JavaScript
var state_province = $('#C_State_Prov option, #C_State_Prov optgroup');
state_province.hide();
$('#C_Country').change(function(){
state_province.hide();
$("#C_State_Prov optgroup[label='"+$(this).find(':selected').html() + "']" )
.children()
.andSelf()
.show();
});