A Pixie tube is a type of obsolete yet beautiful numerical indicator, somewhat like a Nixie tube. Pixie is a trademark of the Burroughs Corporation, and the full name for the device is the "Pixie Position Indicator Tube". The Pixie tube consists of the numbers 0 through 9 cut from a front metal plate, which serves as the anode, and ten cathodes behind it. The numbers are arranged around the center of the tube, instead of being stacked, as in a Nixie tube. A unique feature of the Pixie tube is the ability to light two or more cathodes simultaneously and still get a meaningful display.
/9 0 1\
The envelope of a Pixie tube is probably filled with a mixture of neon and mercury vapor gases under low pressure, like the atmosphere inside a Nixie tube. The tube operates as a cold cathode device, with a minimum operating voltage of 150 volts DC.
As they are no longer in production and no longer installed on new equipment, Pixie tubes are hard to find. If you want to build a clock using Pixie tubes, they should work just fine with any of the clock designs made to use Nixie tubes.
Other fun glowing displays include the Nixie tube, Dekatron, VFD, and Numitron.
Photos of a Pixie tube in action can be found at http://electricstuff.co.uk/count.html