iOS 9: Notification Center has two widget columns on iPad in landscape

If you turn your iPad to landscape in iOS 9, it now has two columns for widgets. The left column stays put, it cannot be swiped away or otherwise obscured, so it might be a good place for your Most Important Widgets. The right column shares space with the Notifications tab.

You can organize these by tapping the standard Edit button at the bottom of Notification Center. It has two separate spaces to which you can drag widgets.

In portrait, the right column widgets simply stack below the left.