Page "Chord (aircraft)" Paragraph 5

Most wings do not have a rectangular planform so they have a different chord at different positions along their span.

To give a characteristic figure which can be compared among various wing shapes, the mean aerodynamic chord, or MAC, is used.

The MAC is somewhat more complex to calculate, because most wings vary in chord over the span, growing narrower towards the outer tips.

This means that more lift is generated on the wider inner portions, and the MAC moves the point to measure the chord to take this into account.