We say :

The Roland K-25M is a 25 note keyboard for use with all of the various Boutique synths produced by Roland. The synths are interchangeable and can be linked together, so there is probably no need to buy more than one keyboard for your collection. Also, remember that you can use any MIDI keyboard to control your synthesizers, but if you want something small and portable then the K-25M is the perfect solution. 

Roland say : 

The K-25M is a 25-key, velocity sensitive keyboard designed for use with the Roland Boutique series. Measuring less than 12″ across, the K-25M acts as a dock for any Roland Boutique module, and once in place the module can be tilted at two different angles for easier viewing and sound programming, or can be laid completely flat.