  Triple-GEM detector technology was recently selected by CMS for a part of the
upgrade of its forward muon detector system as GEM detectors provide a stable
operation in the high radiation environment expected during the future
High-Luminosity phase of the Large Hadron Collider (HL-LHC). In a first step,
GEM chambers (detectors) will be installed in the innermost muon endcap station
in the $1.6<\left|\eta\right|<2.2$ pseudo-rapidity region, mainly to control
level-1 muon trigger rates after the second LHC Long Shutdown. These new
chambers will add redundancy to the muon system in the $\eta$-region where the
background rates are high, and the bending of the muon trajectories due to the
CMS magnetic field is small. A novel construction technique for such chambers
has been developed in such a way where foils are mounted onto a single stack
and then uniformly stretched mechanically, avoiding the use of spacers and glue
inside the active gas volume. We describe the layout, the stretching mechanism
and the overall assembly technique of such GEM chambers.
