When using horizontal navigation, any visible pages directly beneath the topmost page of your site will appear in the local navigation as tabs. If you have a lot of Tabs these can wrap around onto a second line.
To prevent tabs spreading onto two lines you should consider moving some pages to fit further down the site hierarchy, with a view to collating them underneath the six main tab sections. In the meantime you are able to 'hide pages', these pages will still exist but they will not appear in the local navigation.
Once tabs in the horizontal navigation overlap, this breaks the visual connection between the main section tabs and sub-pages within them, making the navigation, and web site, harder for people to use.
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