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A contrastive investigation of the symmetry aspects of phonons in monolayer chromium trihalides and their Janus structures Y$_3$-Cr$_2$-X$_3$ (X, Y = F, Cl, Br, I) by group theory is presented. We first classify all phonons at the Brillouin-zone center ($Γ$) into the irreducible representation. Then the infrared and Raman activity of optic phonons, Raman tensors, and the possible polarization assignments of R active phonons are predicted. Base on these results, we clarify the the discrepancy about the Raman activity o optic modes in monolayer CrI$_3$. Besides, we find that the Raman and infrared spectra for X$_3$-Cr$_2$-X$_3$ are exclusive, whereas that for Janus Y$_3$-Cr$_2$-X$_3$ are coincident. This distinction is vital for optic spectra identification of Janus Y$_3$-Cr$_2$-X$_3$ monolayer from X$_3$-Cr$_2$-X$_3$ monolayer. In addition, we derive the symmetry-matched phonon eigenfunctions and corresponding schematic representations of the eigenvectors for both F$_3$-Cr$_2$-I$_3$ and I$_3$-Cr$_2$-I$_3$ monolayer, which demonstrate intuitively the origin of phonon chirality and magnetism. At last, our analysis indicates that the spin-phonon coupling, the magneto-optical effect of infrared and Raman active phonons, and phonon chirality should be observed in Janus Y$_3$-Cr$_2$-X$_3$ monolayer as that and even easier than that in X$_3$-Cr$_2$-X$_3$ monolayer. Our work provides a detailed guiding map for experimental characterization of Y$_3$-Cr$_2$-X$_3$ monolayer, and also reveals important effects of optic phonons in Janus Y$_3$-Cr$_2$-X$_3$ monolayer.