  As part of its HL-LHC upgrade program, the CMS Collaboration is developing a
High Granularity Calorimeter (CE) to replace the existing endcap calorimeters.
The CE is a sampling calorimeter with unprecedented transverse and longitudinal
readout for both electromagnetic (CE-E) and hadronic (CE-H) compartments. The
calorimeter will be built with $\sim$30,000 hexagonal silicon modules.
Prototype modules have been constructed with 6-inch hexagonal silicon sensors
with cell areas of 1.1~$cm^2$, and the SKIROC2-CMS readout ASIC. Beam tests of
different sampling configurations were conducted with the prototype modules at
DESY and CERN in 2017 and 2018. This paper describes the construction and
commissioning of the CE calorimeter prototype, the silicon modules used in the
construction, their basic performance, and the methods used for their
calibration.
