Work out the angles in each quadrilateral you make.

Perhaps checking that the four angles do add up to ${360}^{\circ }$ !

Now, what other relationships you can see ?

Many thanks to Geoff Faux who introduced us to the merits of the 9 pin circular geo-board.
The boards, moulded in crystal clear ABS that can be used on an OHP (185 cm in diameter), together with a teacher's guide, are available from Geoff at Education Initiatives