What are alt lines?
Alt lines (alternate lines) are extra numbers for the same stat. A lower line is easier to clear and pays less; a higher line is harder and pays more. Books list them as a ladder, like 15+, 20+, 25+ points.
An example ladder
Say a player's main points line is 24.5 at -110. The book might also offer 20+ points at -250, 25+ at -105, 30+ at +210 and 35+ at +500.
"20+" is the same as Over 19.5. Each rung is its own bet with its own price.
Why the price changes
The price follows the chance. 20+ at -250 implies about 71%; 30+ at +210 implies about 32%. Both include the book's margin.
Things to watch
Short prices add up fast in parlays: several -400 legs still lose often when combined.
Margins on far-off rungs are often bigger than on the main line. Compare the rung's price with how often the player has cleared that number, and with his projection range.
Questions
Is 25+ the same as Over 24.5?
Yes. A whole-number milestone like 25+ means 25 or more, which is the same as Over 24.5.
Do alt lines have more vig?
Often, yes, especially the far rungs. One-sided ladders also have no Under to compare, so the fair price is harder to work out.