Toilet (3" drain)
                   +--+
                   |  |
                   |  |
                   |  |
                   |  |
       +-----------+  +-------------------------------+
                   |  |
   Vent?           |  +--------------+         (wall)
     +-----------> | X  -----------+ |
                   |   /           | |
       +-----------+  +------------+ +---------------+
                   |  |            | |
                   |  |            | |
                   |  |            | |
                   |  |            | |
                   |  |            | |
        To main +  |  |            | |
        drain   |  |  |            +-+
                |  |  |           Tub (2" drain)
                |  |  |
                |  |  |
                v  |  |
                   |  |