The UTM Grid
This map/picture is copyrighted by Peter H. Dana
UTM longitude zone

The UTM system divides the surface of the Earth between 80° S latitude and 84° N latitude into 60 zones, each 6°
of longitude in width and centered over a meridian of longitude. Zones are numbered from 1 to 60. Zone 1 is
bounded by longitude 180° to 174° W and is centered on the 177th West meridian. Zone numbering increases in an
easterly direction.

Each of the 60 longitude zones in the UTM system is based on a Transverse Mercator projection, which is capable
of mapping a region of large north-south extent with a low amount of distortion. By using narrow zones of 6° in width,
and reducing the scale factor along the central meridian to 0.9996, (a reduction of 1:2500) the amount of distortion
is held below 1 part in 1,000 inside each zone. Distortion of scale increases to 1.0010 at the outer zone boundaries
along the equator.

The reduction in the scale factor along the central meridian creates two lines of true scale located approximately
180 km on either side of, and approximately parallel to, the central meridian. The scale factor is too small inside
these lines and too large outside of these lines, but the overall distortion scale inside the entire zone is minimized.
(Source:
Wikipedia)