Lyre-playing may have been deferred, as a harder subject, till the boy had learned letters for several years; but the seven-stringed lyre, with the simple old Hellenic music, was probably not a very difficult instrument to master. The chief factor which determined the arrangement of subjects in an ordinary family was no doubt the paidagogos. If there was only one son, he could go to whatever school his parents pleased; but if there were several, elders and youngers had all to go to the same school at the same time, for there was only one paidagogos to a whole family as a rule, and he could never allow any of his charges to go out of his sight. That the three subjects were usually taught simultaneously may be inferred from a passage of Xenophon. “In every part of Hellas except Sparta,” he says, “those who claim to give their sons the best education, as soon as ever the child understands what is said to him, at once make one of their servants his paidagogos, and at once send him off to school to learn letters and music and the exercises of the Palaistra.”[128] The emphasis upon the word “at once” certainly implies that the three subjects began simultaneously. On the vases letters and music are seen being taught side by side in the same school; this was a convenient and natural arrangement. Writing-tablets and rulers are also seen suspended on the walls of music-schools and palaistrai, and lyres on the walls of letter-schools[129]; which suggests that the boys went from one building to another in the day, taking their property with them. Plato states that three years apiece was a reasonable time for learning letters and the lyre.[130] The eight years between six and fourteen, the ordinary time devoted at Athens in the fourth century to the primary triple course, would give space for these six years, with two years to spare for elaboration. Gymnastics are meant to go on during the whole period in Plato, and so do not require a special allowance of time to themselves.