Lowball is a general term for a category of poker variants that turn the typical rank of poker hands on its head—instead of the best poker hand winning, the worst one does! A royal flush in a lowball game would be beaten by any other hand. Any poker variant can be played with lowball rules, and some games split the pot between the best conventional (high) hand and the best low hand. However, there are a few different ways that the lowest hand can be reckoned, which need to be established before you start playing.
Ace-to-five lowball, or California lowball, is probably the simplest way of determining the low hand, and the one most commonly used, including in casinos. In this variant of the game, straights and flushes are ignored for the purposes of determining hand ranking. Aces are considered low. Therefore, the lowest (and therefore best) possible hand is A-2-3-4-5, which is also called the wheel or bicycle. (Because the bicycle is also a straight, it may well take both the high and low halves of the pot in split-pot games.) Note that pairs, three-of-a-kinds, and so forth do still count as hands, and will therefore be ranked higher (and therefore worse) than unpaired hands, even if they contain high cards.
Deuce-to-seven lowball, also known as Kansas City lowball, takes straights and flushes into consideration when ranking hands, and aces count high. Thus, the lowest possible hand is 2-3-4-5-7 (because 2-3-4-5-6 forms a straight).
Ace-to-six lowball is the least commonly-used variation, serving as sort of a middle ground between the two variants listed above. It is essentially deuce-to-seven lowball, except aces are low, so the lowest possible hand is A-2-3-4-6.
Lowball hands are often quoted as their highest card. 8-7-5-3-2 may be called simply “an eight”. If there are multiple hands in play with the same highest card, they can be further disambiguated by the second-highest card, e.g. “an 8-7”.
Some split-pot games involving low hands may stipulate that a hand must contain cards below a certain rank. For instance, if a low hand is required to be “8 or better”, as in Omaha Hi-Lo 8 or Better, all of the cards within the low hand must be an 8 or lower. In such games, if the lowest hand possible among the active players does not meet the requirements, the pot is simply not split, with the entirety being awarded to the player that won with the highest hand.
To determine which hand is lowest and therefore best, start with the highest card. Whichever hand has the lowest high card will win. If there are ties, the next-highest card is compared, and so on until the tie is broken. If you get all the way down to the lowest card without being able to break the tie, the pot is simply split.
Wild cards, usually jokers, can be included in the game, especially ace-to-five lowball. Wild cards generally become the lowest card possible without forming a pair. For example, 7-5-4-★-2 will count the joker as a 3 because counting it as a 2 would form a pair.
In split-pot games, if the pot cannot be split evenly, it is customary to award the odd amount to the winner of the high hand.