The origin of this Portuguese peculiarity isn't clear, but it is mentioned in the oldest stories. Some suggest a biblical root in the story of the doors of the Jews being written on in blood to protect them from the rain of fire. Others refer to the Inquisition, when people accused of practising the dark arts had their homes marked with blood. Some say that red is the colour of the devil's writing. Others point out that amounts owed were tradionally written out in red.

Whatever the origin, there is no question that writing in red is considered a serious offence.