The temple is based on horizontal and vertical lines which meet at right angles. However, it turns out that the human eye distorts these lines when looking at large constructs. Long horizontal lines, for example, appear to sag in the middle, while two parallel vertical lines seem to spread away from each other as they go up. To counter the effect, the Greeks replaced the most prominent horizontal line by a line that bows upwards in the centre. Every other horizontal line then has to be made parallel to this newly introduced curve. The columns of the Parthenon were made to lean together at the top, just a few degrees, to make them seem parallel. (See [3] in the reading list below for more information.)