  Studies of longitudinal correlations of the charged-particle multiplicity in
heavy-ion collisions have provided insights into the asymmetry and fluctuations
of the initial-state collision geometry. In addition to the expansion of the
medium in the transverse direction, commonly quantified using Fourier
coefficients ($v_{n}$), the initial geometry and resulting longitudinal
expansion as a function of azimuthal angle enable us to better understand the
full 3-dimensional picture of heavy-ion collisions. In these proceedings,
azimuthal correlations of the longitudinal structure of charged-particle
multiplicity are reported for Pb-Pb collisions at a nucleon-nucleon
center-of-mass energy of 2.76 TeV. The azimuthal angle distribution is divided
into regions of in-plane and out-of-plane with respect to the second-order
event plane, and the coefficients of Legendre polynomials are estimated from a
decomposition of the longitudinal structure of the charged-particle
multiplicity at midrapidity ($|\eta| < 0.8$) on an event-by-event basis in each
azimuthal region for different centralities. Correlations between the
coefficients of various orders in different azimuthal regions are studied and
exhibit collective features of longitudinal structure in the azimuthal
direction. The results are compared with HIJING and AMPT simulations.
