What is a Caltri?

One of these.

It consists of three 15*15*150mm boxwood rods fixed mutually at right angles, varnished and then painted red (x), green (y) and blue (z) as you see. It is useful to engineers and scientists considering three-dimensional co-ordinate transformations. You can do this with the thumb and first two fingers of your right hand, indeed this is traditional, but the caltri is easier! Obviously one could also do a left-handed version if needed but usually you will not want this.

Why 'Caltri'? well it looks a bit like an abbreviated caltrop (albeit with 90 degree rather than 120 degree angles) and adding the 'i' seemed a good idea to represent the three arms.