  This paper explores the relationship between closed curves on surfaces and
their intersections. Like Dehn-Thurston coordinates for simple curves, we
explore how to determine closed curves using the number of times they intersect
other curves. We construct and study $k$-equivalent curves: these are distinct
curves that intersect all curves with $k$ self-intersection points the same
number of times. We show that such curves must intersect all simple curves in
the same way, but that all other possible implications fail. Our methods give a
quantitative approach to a theorem of Otal which shows that curves are
determined by their intersection with all other curves. In the opposite
direction, we show that non-simple curves can only be distinguished by looking
at their intersection with infinitely many curves.
