  We have used the Australian Long Baseline Array (LBA) to produce
milliarcsecond images of five sites of methanol maser emission at 6.7 GHz.
These are all sites that have linear morphologies at arcsecond resolutions,
which have been hypothesised to be due to the masers forming in edge-on
circumstellar disks. We find that a simple disk model cannot explain the
observations. We discuss various alternatives, and suggest a new model which
explains how linear velocity gradients can be produced in methanol masers that
arise in planar shocks propagating nearly perpendicular to the line of sight.
