  We show that one can measure the effects of microlens parallax for binary
microlensing events with three well-measured peaks -- two caustic crossings
plus a cusp approach, and hence derive the projected Einstein radius. Since the
angular Einstein radius is measurable from finite source effects for almost any
well-observed caustic crossing, triple-peak events can yield the determination
of the lens mass. We note that, to a certain extent, rotation of the binary can
mimic the effects of parallax, but it should often be possible to disentangle
parallax from rotation by making use of the late-time light curve.
