Absorption by associated ionised gas in the SNR 3C 396. Comparison between the 74 MHz/1.4 GHz radio spectral index distribution over 3C 396 (the same displayed in Fig. 2) and the filaments emitting in ionic iron line emission (image kindly provided by Dr. Lee, Y.-H.). The flattest spectral index feature is located where the bright [FeII] near-IR filaments are observed. Contours from the 74-MHz VLSSr image at levels 0.64, 1.7, 2.5, and 3.6 Jy beam⁻¹ are included for reference.