We report on the performance of a large micro-pattern detector
with two gas electron multiplier foils (GEM) and a two-layer
readout structure at ground potential. The two readout layers
each have a 406 micro-m pitch and cross at an effective angle of
6.7 degrees. This structure allows for two orthogonal
coordinates to be determined. Using a muon beam at CERN together
with a silicon tracking system, the position resolutions of the
two coordinates are measured to be 50 micro-m and 1 mm
respectively (1 stand.dev.). The muon detection efficiency for
the two-dimensional space points reaches 96%. The detector was
found to be well operational over a wide range in the settings
of the different electrical fields
