Canção de intervenção is a restrained, composed song accompanied by acoustic and Portuguese guitar. Rather than shouting slogans, it sounds like a quiet resolve grounded in folk and poetic tradition. Metaphor and allegory are woven into the lyrics to slip past the censor, around themes of land, freedom and solidarity.

In the 1960s, under Salazar's dictatorship (the Estado Novo), the 'intervention song' arose to resist censorship and repression. Drawing on the student-song tradition of the University of Coimbra, Zeca Afonso and Adriano Correia de Oliveira joined folk melody to poetry and sang criticism of the regime within it.

Listen for the suppressed anger and resolve hidden inside an unraised, quiet delivery. The delicate ring of the Portuguese guitar and the plain weight of male choirs evoke the traditions of fado and student song. Knowing the allegories in the lyrics (vampires standing for the rulers, for instance) lets the criticism rise up beneath the surface calm.

Zeca Afonso's 'Grândola, Vila Morena' (1971) is essential, a song that carries the very history of becoming a revolution's signal. To hear the allegorical criticism, turn to his 'Os Vampiros' (1963).

In the early hours of 25 April 1974, rebel officers played 'Grândola, Vila Morena' on the radio as the signal to begin their uprising. The broadcast of a single song triggered the bloodless Carnation Revolution, a rare event in world history.