One surface, one edge. The light goes round twice.

A band given a half turn and joined back to itself has only one side. Set a finger on the lit line and follow it. You will pass the point you started from, find yourself on what looked like the other face, and only come back to the beginning after a second turn. That is not a trick of the eye. It is a property of the shape, and the light inside obeys the same rule, which is why the lit line is twice as long as the ring is round.

The band is built in six segments and closed by hand into one continuous loop, and it stands on a base of cast concrete that holds it still. There is no inside to hide the light in, so from every angle you see it leaving a single unbroken line.