The H-mode develops in conditions of low and controlled recycling. The electron and ion collisionality parameters at the plasma periphery are close to, but above, one in the L-phase and drop below one in the H-phase. The realization of the H-phase is easier in the single-null than in the double-null configuration which has higher power and density thresholds. The increased H-mode accessibility of the single-null configuration requires the ion grad B-drift direction to be towards the stagnation point. The experimental observations are in agreement with a recent theory of the H-transition by Hinton.