  This paper revisits the resonant behavior of a harmonically-forced Duffing
oscillator with a specific attention to phase resonance and to its relation
with amplitude resonance. To this end, the different families of resonances,
namely primary (1:1), superharmonic (k:1), subharmonic (1:{\nu}) and
ultra-subharmonic (k:{\nu}) resonances are carefully studied using first and
higher-order averaging. When the phase lag is calculated between the k-th
harmonic of the displacement and the harmonic forcing, this study evidences
that phase resonance occurs when the phase lag is equal to either {\pi}/2
(phase quadrature) or 3{\pi}/4{\nu}.
