Point spread
A handicap market: the favorite must win by more than the spread, the underdog can lose by less than it, and both sides are usually priced near −110.
A spread of −1.5 on a baseball favorite means it must win by two or more runs; +1.5 on the underdog wins if it loses by one or wins outright. Books set the number so that action splits roughly evenly, then charge vig on both sides.
Spreads are where cross-book disagreement is most common, because books move the number and the price independently. Two books quoting the same line at different prices is a classic arbitrage; two books quoting different lines is a middle. ArbMonster only pairs identical lines for arbitrage and reports spread margins per book.
Padres −1.5 at −105 on one book and Twins +1.5 at +105 on another: implied 51.2% + 48.8% = 100.0% — a coin-flip, not an arb. At −105 / +112 it drops under 100% and becomes one.
Measured on ArbMonster: Why spreads and totals see more arbitrage