  The study of open string tachyon condensation in string field theory can be
drastically simplified by making an appropriate choice of coordinates on the
space of string fields. We show that a very natural coordinate system is
suggested by the connection between the worldsheet renormalization group and
spacetime physics. In this system only one field, the tachyon, condenses while
all other fields have vanishing expectation values. These coordinates are also
well-suited to the study of D-branes as solitons. We use them to show that the
tension of the D25-brane is cancelled by tachyon condensation and compute
exactly the profiles and tensions of lower dimensional D-branes.
