What Is an Ad Tag, and What Happens When It Fires

An ad tag is a small piece of code you paste into your page. When somebody opens the page, the tag asks an ad server for an advertisement and puts whatever comes back into that spot.

That is the whole thing. It is usually three or four lines, it goes in once, and the rest of this post is what happens inside those lines.

What one looks like

Different platforms word it differently, and every one of them is a version of the same two ideas: here is an empty box, and here is the code that fills it.

<div id="pb-zone-4821"></div>
<script src="https://yournetwork.example.com/serve.js" async></script>

The div is the empty box, and it is where the ad will appear. The number in it is the zone identifier: the thing that tells the server which of your slots is asking. The script is the code that does the asking.

What happens when it fires

Usually under 100 milliseconds. It happens once per slot, not once per page.

What the tag sends

Less than people assume. To choose an ad the server needs to know which slot is asking, what size it is, roughly where the reader is, what kind of device they are on, and what the page is about.

For a contextual setup that is the complete list. It does not send who the reader is, because it does not choose on who the reader is.

What comes back

A creative, or nothing. Nothing is a real answer: if no campaign cleared the floor price for that slot, the server says so and the fallback takes over. See what a passback does.

Where to put it

Putting it in without fear

The thing publishers actually worry about is breaking their site, so it is worth saying plainly what a tag can and cannot do.

• It is additive. It adds a box. It does not modify your layout, your styles or anything else on the page.
• It is asynchronous. The async attribute means your page carries on loading while the ad is being fetched, so a slow ad server cannot hold up your article.
• Removing it undoes it. Delete the two lines and your page is exactly as it was.

Where it goes depends on your platform: a Custom HTML block in WordPress, a theme template in Shopify, or directly into the page for plain HTML. If you use a caching plugin, clear the cache afterwards, which is the single most common reason somebody thinks the tag did not work.

When nothing appears

In rough order of likelihood:

An empty slot with no fallback set looks identical to a broken tag, which is why setting a fallback before you paste anything is worth the extra minute.