A more thorough explanation:

Antiquus et novus extentus is a Latin term used in Scots law. It refers to the old and new extent of land valuation for tax purposes. The old extent was assessed in the year 1280, while the new extent was assessed several times after that date.

If a piece of land was valued at £100 in 1280 and its value increased to £200 in 1300, then the old extent would be £100 and the new extent would be £200.

This term is important in Scots law because it helps determine the amount of tax that should be paid on a piece of land.