Tentatively, PEA n = 2 samples display at least local noncentrosymmetry, which may be due to the disorder in the cations contributing to a noncentrosymmetric structure not observable in SCXRD measurements. The possibility of localized and spatially dependent SHG response could be investigated by future microscopy studies. Other researchers have found the disordered nature of PEA n = 1 cations to be an important influence to breaking local degeneracy in the excitonic energy landscape of thin films. Their observations, like ours, suggest the importance of RNH 3 cations in determining the symmetry of the electronic states of 2D-RP perovskites. Our multidimensional harmonic generation method could be extended in the future to measuring excited state symmetries (like those considered in refs. ) by the addition of an optical pump. To further demonstrate the capability of multidimensional harmonic generation to discriminate against mPL, we grew a single crystal of n = 3 (BA) 2 (EA) 2 Pb 3 I 10 , which has recently been identified as a 2D-RP perovskite that is ferroelectric at room temperature. As shown in Figure , this ferroelectric 2D-RP perovskite has intense mPL that could obfuscate a conventional SHG measurement. However, because we observe an unambiguous feature (highlighted by a blue line) whose output frequency tracks twice the input frequency, we can be assured that (BA) 2 (EA) 2 Pb 3 I 10 is indeed SHG active, and thus non-centrosymmetric, and therefore possibly a ferroelectric material at room temperature. The confirmation that a 2D-RP perovskite with a butylammonium spacer cation and ethylammonium A-site cation is non-centrosymmetric is especially of note because the analogous (BA) 2 (MA) 2 Pb 3 I 10 RP perovskites with MA in the perovskite cage have been confirmed to be centrosymmetric (see results and discussion above). This observation indicates that small permutations in both the A-site and RNH 3 spacer cations can influence the inversion symmetry of the entire 2D-RP perovskite crystal structure. We have shown that multidimensional harmonic generation can be used to definitively measure and clarify the SHG and THG response of halide perovskite materials, even in the presence of a strong mPL background. By scanning and correlating both excitation and emission frequencies, we unambiguously assess whether a material supports SHG. This methodology is systematically applied to single crystals of seven 2D-RP perovskites with different spacer cations, n values, and A cations. The lack of SHG in n = 2 and n = 3 2D-RP perovskites with BA spacer cations definitively indicates that these materials have a centrosymmetric crystal structure instead of the previously reported non-centrosymmetric structures based on inconclusive SHG measurements. Observation of SHG from n = 2 and n = 3 2D-RP perovskites with PEA and TPMA spacer cations demonstrate that bulky, aromatic spacer cations can template the lead iodide network creating materials that lack inversion symmetry. There remains a contradiction between our SCXRD analysis that shows the (PEA) 2 (MA)Pb 2 I 7 crystal is centrosymmetric, and the observation of SHG. This contradiction should be investigated by future microscopy studies. We further showed that multidimensional harmonic generation confirms the noncentrosymmetry of the recently reported ferroelectric 2D-RP perovskite (BA) 2 (EA) 2 Pb 3 I 10 even in the presence of bright mPL. This work establishes multidimensional harmonic generation as a definitive technique to reveal the SHG properties of a material. Our results also demonstrate that subtle variations in A-site and RNH 3 spacer cations can be used to control the overall crystal symmetry of 2D-RP perovskites as strategies to engineer and design next generation ferroelectric materials.