Michelangelo – Prisoners (c 1519-34)

Michelangelo's Prisoners, or Slaves, were begun for the tomb of Pope Julius II but never finished. In its entirety – including the Dying and Rebellious Slaves in the Louvre and the statue of Moses on the final, reduced version of the tomb eventually erected in Rome – this constitutes the greatest unfinished masterpiece in the world. Yet Michelangelo did not leave things unfinished out of laziness. It is an aesthetic choice. The tragic power of these prisoners as they struggle to emerge out of raw stone is an expression of the human condition that equals Shakespeare's Hamlet.
• Accademia Gallery, Florence

Velázquez – Las Meninas (c 1656)

The king and queen stand where you are standing, in front of a gathering of courtiers. Velazquez looks from the portrait he is painting of the royal couple. The infanta and her retinue of maids (meninas) and dwarf entertainers are gathered before the monarch. In the distance, a minister or messenger is at the door. In a bright mirror, the royal reflection glows. This painting is a many-layered model of the world's strangeness.
• Prado, Madrid