  A novel hadron calorimeter is being developed for future lepton colliding
beam detectors. The calorimeter is optimized for the application of Particle
Flow Algorithms (PFAs) to the measurement of hadronic jets and features a very
finely segmented readout with 1 x 1 cm2 cells. The active media of the
calorimeter are Resistive Plate Chambers (RPCs) with a digital, i.e. one-bit,
readout. To first order the energy of incident particles in this calorimeter is
reconstructed as being proportional to the number of pads with a signal over a
given threshold. A large-scale prototype calorimeter with approximately 500,000
readout channels has been built and underwent extensive testing in the Fermilab
and CERN test beams. This paper reports on the design, construction, and
commissioning of this prototype calorimeter.
