J–K_s vs. K_s colour-magnitude diagram of the four observed fields in W51 showing the positions of all the stars in each field (grey points) and the sub-clusters (black points) defined by the circles on Fig. 2. The red triangles indicate the spectroscopically classified massive stars. The red triangle in panel b is the position of LS1 (note the different y-axis in this panel). The red squares in panel d are the massive stars classified by Figuerêdo et al. (2008) and Barbosa et al. (2008). The blue dashed vertical line represents the un-reddened isochrone for main sequence stars with an age of 1 Myr (Ekström et al. 2012; Yusof et al. 2013). The blue solid line is the reddening-free pre-main-sequence isochrone of Tognelli et al. (2011). The red lines represent the same isochrones, but reddened with A_Ks = 0.45 mag to highlight the foreground extinction.