Both rules apply only to even-money bets (Red/Black, Even/Odd, High/Low) and only when zero lands.
1. La Partage ("the divide").
You have $50 on Red. Zero lands. Instead of losing the lot, the casino returns 50% of your stake, $25, and keeps the other $25. Your loss on a zero is halved, which halves the long-run edge on that bet from 2.70% to 1.35%.
2. En Prison ("in prison").
You have $50 on Red. Zero lands. The bet is frozen for one additional spin rather than settled.
- Next spin lands Red → your original $50 is returned, with no winnings.
- Next spin lands Black → the $50 is lost.
- Next spin lands zero again → house rules vary; commonly the bet stays imprisoned for another spin, or is halved again.
The expected value of En Prison is effectively the same as La Partage, 1.35% on even-money bets. Which version you meet depends on the operator and the table, so check the rules panel before you sit down or click "play".
Practical takeaway for short sessions: if you are planning to bet even money for a small number of spins, a French table with La Partage gives you the mathematically best version of roulette that exists in mainstream casinos. That is a real, verifiable improvement, unlike any progression system.