Vig, hold and the no-vig fair line

What the number means

Convert every price in a market to a probability and add them up. A market with no margin would total exactly 1.00. Real markets total more, and the excess is the operator's margin. Two sides at -110 total 1.0476, an overround of 4.76%; expressed as a share of the money taken on perfectly balanced action it is 4.55%, which is the hold.

Overround and hold are different denominators for the same margin, and quoting one when you meant the other is the most common arithmetic mistake in betting writing. Overround divides by the fair book; hold divides by the total staked. Hold is always the smaller number.

The no-vig line is what the price would be if the margin were removed proportionally. It is the cleanest available read on what the market thinks, and it is the number worth comparing your own estimate against, not the posted price.

Where it misleads

Proportional devigging assumes the margin is spread evenly across the outcomes. It usually is not. Longshots typically carry more of the margin than favourites, so on a lopsided market this method flatters the longshot's fair price. Alternative methods exist (multiplicative, additive, power, Shin) and they disagree most exactly where it matters most, on heavy favourites and three-way markets with a big price on the draw. Treat the fair line as an estimate with a method attached, not a fact.

Devigging one book also tells you about one book. Hold varies by operator, by sport, by market and by how close it is to kick-off, which is why the same game can be priced meaningfully differently in two places at the same moment.