  The recent discovery of the $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) material family
offers an exciting opportunity to investigate the interplay of correlations,
topology, and superconductivity in kagome metals. The low energy physics of
these materials is dominated by an unusual charge density wave phase, but
little is understood about the true nature of the order parameter. In this
work, we use a combination of ultrafast coherent phonon spectroscopy and
first-principles density functional theory calculations to investigate the
charge density wave order in CsV$_3$Sb$_5$. We find that the charge density
wave is the result of a simultaneous condensation of three optical phonon modes
at one $M$ and two $L$ points. This distortion can be described as
tri-hexagonal ordering with an interlayer modulation along the $c$ axis. It
breaks the $C_6$ rotational symmetry of the crystal and may offer a natural
explanation for reports of uniaxial order at lower temperatures in this
material family.
