  Simulations in lattice gauge theory suggest that the formation of a flux tube
between quark and antiquark leads to quark confinement. It is conjectured that
the infrared behaviour of the flux tube is governed by an effective string
theory and simulations show good agreement between lattice data and its
predictions. To next-to leading order ($R^{-3}$) in the inverse $q\bar{q}$
separation $R$ the effective string theory is equivalent to Nambu-Goto string
theory. For the open flux tube in three dimensions corrections appear at order
$R^{-4}$. We compare these predictions to high-accuracy measurements of the
groundstate energy of the flux tube in 3d SU(2) and SU(3) gauge theory and
extract the coefficient of the leading order boundary term in the effective
action.
