I'm using Joomla, and as far as I can tell I have done everything right for my template, but can't get Module positions to work

I have the correct XML file setup I think and this is my HTML tag:

```
    <jdoc:include type="modules" name="topnav">

```

The page is X if you look at the page source it is rendering that tag - I would expect that to disappear, and to be replaced with the content I have told the Joomla backend to put there?

Very confused!