Couldn't find a definitive answer, but I've cobbled together this likely scenario. (If I'm wrong, someone should post the truth-- wish you would have posted it when AV2 asked to start with!!
) Anyway, here goes:
It's all about baseball. Football, hoops, and American Line hockey all have pointspread odds of one form or another and "implied odds" (a football or hoops side or total is usually -110 either way; pucks is even either side on a split line). Bases is the only sport where the majority of bets are moneyline.
Anyway, odds in bases used to be expressed based around a $5 bet or "time". Favourites were listed first so you might see:
Cardinals 6-7 Dodgers
So you'd bet $7 to win $5 on the Cards or $5 to win $6 on the underdog Dodgers. As the game grew more lopsided, the gap increased and you might see:
Yankees 9-11 Tigers
which means $11 on Yanks to win $5 or $5 on Tigers to win $9.
Anyway, competition between BMs being what it was, some bookmakers decided to cut the vig in half (go to a ten-cent, or "DIME" line) on the condition that punters bet a minimum of $100 on the dog to enough to win $100 on the favourite. [Logic: Juice lost by making the lines closer is regained through volume business.]
Anyway, a Red Sox/Angels game with odds of -$145/+$135 would look like
Red Sox 6.75-7.25 Angels
on the old system. Gross! So, since punters playing into these lower vig lines had to stake $100 or more, they just expressed the numbers based around a $100 bet, with the minus sign for laying and the plus sign for taking. (This became imperative as some books started adjusting lines in increments of less than 5 cents...imagine trying to express -$138/+$128 in the old system!)
And that's how it came to pass.

It's all about baseball. Football, hoops, and American Line hockey all have pointspread odds of one form or another and "implied odds" (a football or hoops side or total is usually -110 either way; pucks is even either side on a split line). Bases is the only sport where the majority of bets are moneyline.
Anyway, odds in bases used to be expressed based around a $5 bet or "time". Favourites were listed first so you might see:
Cardinals 6-7 Dodgers
So you'd bet $7 to win $5 on the Cards or $5 to win $6 on the underdog Dodgers. As the game grew more lopsided, the gap increased and you might see:
Yankees 9-11 Tigers
which means $11 on Yanks to win $5 or $5 on Tigers to win $9.
Anyway, competition between BMs being what it was, some bookmakers decided to cut the vig in half (go to a ten-cent, or "DIME" line) on the condition that punters bet a minimum of $100 on the dog to enough to win $100 on the favourite. [Logic: Juice lost by making the lines closer is regained through volume business.]
Anyway, a Red Sox/Angels game with odds of -$145/+$135 would look like
Red Sox 6.75-7.25 Angels
on the old system. Gross! So, since punters playing into these lower vig lines had to stake $100 or more, they just expressed the numbers based around a $100 bet, with the minus sign for laying and the plus sign for taking. (This became imperative as some books started adjusting lines in increments of less than 5 cents...imagine trying to express -$138/+$128 in the old system!)
And that's how it came to pass.
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