By a neck
Definition 1 of 1
Pro Tip 1/3Racing Origin
The phrase originates from horse racing, where a 'neck' is the literal margin of victory. 
He's an avid gambler and often bets on horses that win by a neck.
Pro Tip 2/3Close Competitions
Use 'by a neck' to describe very close competitions, not just races, to emphasize how narrow the win was. 
She won the spelling bee by a neck, with the final word tipping the scale.
Pro Tip 3/3Figurative Use
The phrase is often used figuratively, meaning it's not about actual physical distances. 
The incumbent politician won the election by a neck, just a few votes.