History & CultureMental model
A coin in a grave doesn't give you a year, only a boundary.
Historians often date not with a point but with two limits. Terminus post quem: if a grave holds a coin from 312, it was dug no earlier than 312 – the coin may have circulated for decades. Terminus ante quem: if Vesuvius ash from 79 covers a house, it stood no later than 79. The truth lies between the two.
Useful: For any date, ask: is this an 'earliest', a 'latest', or actually a point in time?
Source: en.wikipedia.org