If you are using a standard corner unit with a door and a blank, you will need to use a corner post. Corner posts can be either purchased at a fixed size from the "Mouldings" section, or constructed from spare material like panel offcuts for example.

Corner posts space two units out at a right angle, this enables the doors to open correctly as in the diagrams below, and in are important if fitting any unit containing drawers in the corner to allow the drawers to open past any handles you may have fitted.

Below is an example of a standard corner unit, but the same applies to all corner units fitted with a door and a blank (including pull out corner units)

This image shows the components not fitted

This image shows the components fitted

From Above these units look like this:

Components not fitted

Components fitted 

Please note that you do not need corner posts with "L" shaped base units.