Comparison of the J band SINFONI spectra (red) of 2M0141 (L0), OTS44 (M9.5), and KPNO Tau 4 (M9.5) to R ~ 2000 templates (black) of M9.5–L1 field dwarfs (BRI B0021-0214, C05; 2M0345, ML03; 2MASS J14392836+1929149, C05) classified in the optical, and to the spectrum of a very late type giant (IO Virginis, R09). The flux ratio inside the gray zones (a) and (b) defines the Na index defined by A07 and used in part 3.1.2. The flux ratio of gray zones (c) and (d) defined by G03 measures the depth of the water band longward of 1.3 μm. It can be used to classify young M dwarfs but appears slightly age-dependent at the M–L transition.