M_J and absolute magnitudes versus synthetic colour i′ − z′ from available spectra of brown dwarfs with known parallax. Open circles: dwarfs earlier than M 8. Open squares: M 8 to L4.5 dwarfs. Filled circles: L5 to L9.5 dwarfs. Open triangles: early T dwarfs. Filled triangles: late T dwarfs. The dotted line shows the derived absolute magnitude colour relation valid for late-M to L dwarfs. Error bars are indicated when larger than the symbol.