The LSI-11 Bus Specification has a section on history and folklore, and states
The original backplane to implement the LSI-11 Bus was designed to
accommodate Digital's quad and dual height modules. It became known
as the "Q-Bus" to reflect the quad board form factor. Both LSI-11 Bus
and Q-BUS are Digital trademarks, but are not registered trademarks.
Several bus architecture features of the LSI-11 Bus are protected by
claims in Digital Unibus patent numbers 3,710,324 and 3,815,099. Some
publications may also use the term SUB-UNIBUS, although this is not
When the LSI-11 Bus was expanded to provide 22 address lines, the
terms Q22-Bus and extended LSI-11 Bus were adopted to specify
backplanes [of] that bus [with] the four additional lines.