  Unstable, non-BPS D-branes in weakly coupled ten dimensional string theory
have many mysterious properties. Among other things, it is not clear what sets
their tension, what is their relation to the better understood BPS D-branes,
and why the open string tachyon on them is described by an effective Lagrangian
which suggests that the tachyon corresponds to an extra spatial dimension
transverse to the branes. We point out that the dynamics of D-branes in the
presence of Neveu-Schwarz fivebranes on a transverse R^3 times S^1 provides a
useful toy model for studying these issues. From the point of view of a 5+1
dimensional observer living on the fivebranes, BPS D-branes in ten dimensions
give rise to two kinds of D-branes, which are BPS or non-BPS depending on
whether they do or do not wrap the S^1. Their tensions are related, since from
a higher dimensional perspective, they are the same objects. D-branes localized
on the S^1 have a tachyon corresponding to their position on the circle. This
field is described by the same Lagrangian as that of a tachyon on a non-BPS
D-brane in ten dimensions. Its geometrical interpretation is useful for
clarifying the properties of non-BPS branes in six dimensions. If the lessons
from the six dimensional system can be applied in ten dimensions, the existence
of non-BPS D-branes seems to suggest the presence of at least one extra
dimension in critical type II string theory.
