The jQuery .add() API allows you to tack on of an additional set of selectors to be added to the original selector for manipulation. What this does is search for every instance of a certain element within the document and allow you to add an additional element or set of elements for the same manipulation. It saves on having to create additional lines of code for each selector.

For example: You want all  <div class="quotebox"> elements to have a background color of #FFDD88. In addition to these <div>s, you also want all elements of <p class="quoteline"> to also have this background color.

$("div.quotebox").add("p.quoteline").css('background-color','#FFEECC');

<div class="quotebox">
    <p>As Mark Twain said:</p>
    <p>The secret to getting ahead is getting started</p>
</div>
<p class="quoteline">Age is an issue of mind over matter. If you don't mind, it doesn't matter.</p>

Result of the code

As Mark Twain said:

The secret to getting ahead is getting started

Age is an issue of mind over matter. If you don't mind, it doesn't matter.

The jQuery .add() API can be chained with other APIs as shown in the example. 

For Further Details: http://api.jquery.com/add/