Art and gridlines: the copying of Old Kingdom scenes in later periods

Works of top Old Kingdom artists were considered as examples to emulate by contemporaries and by later generations. The magnificently decorated tombs of Ptahhotep I, Akhethotep and Ptahhotep II probably remained accessible for a long time. Two observations may be made on their scenes: a) certain parts were left in different states of incompletion. Did it aim at demonstrating their techniques?; b) square gridlines were added above sections of completed reliefs, and the same is found e.g. in the tombs of Iyrnery at Giza and Pepyankh-henykem at Meir. The last case suggests that the gridlines were used by Middle Kingdom artists to copy some Old Kingdom masterpieces in Middle Kingdom tombs, or for teaching purposes in the newly established schools.