5U formats:
MU Panels:
source: https://synthesizers.com/technical.html
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Mounting Flanges
Mounting flanges at the top and bottom of each module panel are .4" tall which mates nicely with a 3/8" (.375) mounting surface.
MOTM Panels:
history of MOTM: https://en.wikipedia.org/wiki/MOTM
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width 2U = 3,5" 88,52mm width (normally)
width 3U = 5,25"
other FORMATS:
see here: https://www.synthesizers.com/formfactors.html
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