Fractional differences between MARCS and ATLAS9 flux relative to the ATLAS9 flux. Top panel: spectra shown from T_eff = 3500 K to 8000 K and between 100 nm and 32 µm. A large discrepancy exists in the UV for all temperatures. Middle panel: relative differences in the optical and near-IR, where the offset is usually within 10%. Bottom panel: relative differences are the smallest in the IR between 1 and 32 µm. The large spikes above 15 µm originate from slight spectral mismatches in the resampling.