A late 5th century BC funerary altar from the necropolis of Krannon (Central Greece) depicts a bearded man and a boy on either side of a board with five lines carved on a block. The fact that the man is seated and the horizontal position of the board reveal important information about Greek education and the history of Greek numeracy. This paper analyses the iconography of the relief, the link between the Five Lines game (Pente grammai) and abaci, examines the possible identification of the man as a “pebble arithmetician”, of the boy as a student, and suggests a new reconstruction of the reckoning system operated on an abacus composed of five horizontal lines. A special practical function is proposed for the half-circle at one end of the abacus. This five lines pattern and the related material, especially counters, are considered from a wider perspective, a system of cultural practices associated with boards and counters throughout the Greek world.