The linear polarizer is the standard polarizing filter, and is an essential outside all-purpose filter. It deepens skies and minimizes reflections.
This filter is typically rotated to achieve the required effect; for this purpose you should ensure that your matte box has at least one rotating filter tray, and make sure when purchasing circular filters that you buy a polarizer with a rotating ring.
For more information on polarizers, please see this
polarizers technical data PDF