z′ − J synthetic colour versus i′ − z′ synthetic colour computed from available spectra in the literature. i′ and z′ are computed in the CFHT/MegaCam filters, J is computed for the NTT/SOFI J_s filter. The filled squares show the colours of each brown dwarf. The dashed line shows the mean colour–colour relation. Spectral types are given along this track, indicating the averaged colour of brown dwarfs with a given spectral type. Crosses with error bars show the T dwarfs for which we obtained spectroscopic observations. An arrow indicates no detection in the i′-band, meaning that the i′ − z′ colour is a lower limit.