The Papplewick Arms was a pub frequented by the Lady Ada Lovelace in 1852. It located in Nottinghamshire, near Newstead Abbey, where the Countess took up residence during daytime. She would escape at nights in order to engage in gambling here, together with a regular group of participants. Lovelace habitually sat in her "usual chair", and played cards with such folk as Harry, Ted, George, and Charlie.
On the night following his arrival at Newstead Abbey, the Fourth Doctor deduced that the Papplewick Arms was the likely venue for Lovelace's secretive night-time activities. He arrived soon after she did, and joined her group as the sixth player in their game of 5-card cribbage. The Doctor partnered up with Lovelace, against two other pairs of players. Harry dealt.
Behind the scenes
Though no pub is known to have existed by this name, the Papplewick Arms corresponds to the real-world village of Papplewick, which is indeed located in Nottinghamshire.