How do you decode the symbols hidden in old paintings?

Reading an old painting is a bit like reading a road sign: part of the message is in the picture and part is in a code you have to be taught. Art historians call the study of that code iconography. It identifies, describes and interprets what an image contains, the subjects, the compositions and the details, rather than the artist's style. A haloed man with a lion beside him is not just a man and a cat: a Western viewer can read him as St. Mark.

The codes were not improvised. In Western art a whole system of attributes grew up so that individual saints could be told apart by a standard appearance and the symbolic objects they hold. Most painters were content to copy and slightly modify other people's work, and the clergy who commissioned most art often specified in great detail what they wanted shown. So the symbols you spot were often chosen by the patron, not invented by the painter.

Some paintings are almost visual puzzles. Vanitas paintings, popular among Dutch painters in the 17th century, are formulaic: they use traditional symbols to convey mortality, and skulls were a favourite way of saying that life does not last. But beware of over-reading. In the Arnolfini Portrait the oranges could carry a meaning, yet they could also simply show that the couple could afford very expensive imports. A symbol is a hypothesis, not a fact, so ask what else the object could mean.