Corner connectors
In white or grey, you can find here a series of corner connectors for all the different rail types. Without corner connectors it is actually not possible to obtain a tight corner connection.
These corner connectors can be used for both an inner corner and an outer corner.
The cap of the corner connector also falls slightly over the edge of the rail, so if you have not completely cut the rail at right angles, the lip of the corner connector will ensure that nobody notices this.