Definition
A key number is a final margin that lands often enough to make nearby point spreads more valuable. In NFL betting, 3 and 7 are key numbers because many games finish by a field goal or a touchdown. If a spread moves from +3.5 to +2.5, it crosses the key number 3, which changes the bet more than a move from +5.5 to +4.5.
There is no single universal formula for a key number. Its value comes from how often games land on that exact margin in a specific sport, market, and scoring environment.
Worked Example
An NFL underdog is listed at +3.5 at -110. A bettor risks $110 to win $100 if the team wins outright or loses by 1, 2, or 3 points. If the line moves to +2.5 at -110, that same bet loses when the team loses by exactly 3.
The price is still -110, but the second ticket has worse coverage because it no longer protects the common 3-point margin.
Why It Matters
Key numbers help a bettor judge whether a line move is meaningful. They matter most when shopping spreads, deciding whether to bet now or wait, or comparing alternate lines around common margins.