  Chirality is a fundamental property of electrons with the relativistic
spectrum found in graphene and topological insulators. It plays a crucial role
in relativistic phenomena, such as Klein tunneling, but it is difficult to
visualize directly. Here we report the direct observation and manipulation of
chirality and pseudospin polarization in the tunneling of electrons between two
almost perfectly aligned graphene crystals. We use a strong in-plane magnetic
field as a tool to resolve the contributions of the chiral electronic states
that have a phase difference between the two components of their vector
wavefunction. Our experiments not only shed light on chirality, but also
demonstrate a technique for preparing graphene's Dirac electrons in a
particular quantum chiral state in a selected valley.
