%q allows different delimiters to represent single quoted strings, which doesn't have string interpolation. For ex:

    $ ruby -e 'a=5; puts "[#{a}]"'
    [5]
    $ ruby -e 'a=5; puts %q/[#{a}]/'
    [#{a}]

%Q or just % does interpolation:

    $ ruby -e 'a=5; puts %Q/[#{a}]/'
    [5]
    $ ruby -e 'a=5; puts %/[#{a}]/'
    [5]