Here's a macro for illustration:

     (mac a (x y)
      `(do (prn ',x)
           (prn `,y)))

    (mac b (x y)
      `(do (prn ',x)
           (prn `,,y)))

    arc> (a 1 2)
    1
    Error: "reference to undefined identifier: _y"
    arc> (b 1 2)
    1
    2
    2