The definition
Matching numbers means the major components — typically engine, gearbox and axle — carry the numbers recorded for that chassis when it left the factory.
How it is verified
Verification means reading the stampings on the car and comparing them with a factory certificate or build record — not accepting a seller's description.
What it is worth
On sought-after models the premium is substantial. On others, a documented period-correct replacement engine and honest history can be the better buy.