Extension tubes are an easy to use and convenient way to improve the close up ability of your H system.
There are 3 tubes available in the range:
H 13mm
H 26mm
H 52mm
All 3 are automatic, fully coupled tubes, giving multi mode and all metering options when used with any of the HC and HCD lenses. 
Not only can they be used in combination ('stacking' tubes), but they can be used in conjunction with the  H 1.7x converter.