Pietro Lorenzetti, 1310-1320. Seattle Art Museum.
Saint Peter and Saint Paul rarely, if ever, smile. In fact, do people in paintings ever smile at the baby Jesus? This triptych, however, shows a gentle merriment across the three panels. The triptych also suggests that, even if presented on individual panels, some of this type of painting may have been done of people posing together, rather than alone.
Look at the baby. He is cranky, tired, and about to snuggle into his mother, except that he has been distracted.
Peter is jingling his keys and wiggling the fingers of his right hand. The baby looks back at him. Their eyes connect. The mother is tired, too, but she is trying to keep her pose for Lorenzetti. She starts to glance at the keys.
Now look at Paul. He is grinning, and his eyes focus directly on Peter's keys. The three panels are held together by the four sets of eyes.
If you look at this panel, Peter and Paul's eyes are looking at Christ's hand, and the key and sword are much more prominent. Peter and Paul are the same type, but there is not the flash of a moment's interaction that we have in this triptych.