  In this note, I revisit the problem of computing the entanglement entropy of
a single interval in the ground state of a 2d CFT. I write the leading-order
result in three different ways: once by doing the replica trick with the
$n$-replicated cylinder partition function computed in the ``closed string
channel"; once by computing the $n$-replicated cylinder partition function in
the ``open string channel", where the entanglement entropy can be related to
the density of states in a boundary CFT defined on the interval; and for
holographic CFTs, once as the log of a Plancherel measure for a certain
noncompact quantum group. I comment on the implications for what the
Ryu-Takayanagi area term might be counting in holographic CFTs.
