Primero is Renaissance cardgame that has many similarities to modern day poker. The following is my redaction, based on the description given by Giralamo Cardano in his "On Games of Chance", included in Oystein Ore's Cardano: The Gambling Scholar. As with poker, there are probably thousands of variants; this represents one.
Bidding
In order to bid, the previous bid must be staked.
Then the bid is announced. It consists of three things: the point
total, hand type, and bid amount. The hand type must be higher than
the previously bid hand type or, if it is the same, the point total
must be higher.
The hand types are, in order of increasing rank:
For example, if a player had a 2 of clubs, 3 of diamonds, 4 of spades, and King of clubs, he would have a primero (one of each suit) and his point total would be 12 + 13 + 14 + 10 = 49; thus he could bid "primero 49". Any player after him who bid would have to beat a primero 49, either by naming a higher rank (e.g., Maximus) or a higher point total (e.g., "Primero 59"). If he wanted to bid 5 scudi, his bid in full would be "Primero 59 at 5 scudi".
Note that the point totals include only the total of the cards that make up the hand. For example, a numerus point total could be as low as 20. (If you've seen the earlier editions, this was not specifically pointed out)
Players may deliberately understate their point total or their hand type. For the above, the player can bid no higher than a primero 49, but could claim to have a numerus 49, or a primero 40.
Staking
Staking, or "covering" a previous bid involves putting money
down. No other action is necessary or required. Unlike poker, you
have only to cover the last bid, not all the bids since you last
staked. Thus if the players are Alberto, Bernardo, Cinthio, and
Domenico, and Bernardo bids "numerus 40 at 10 scudi", then
Cinthio covers and bids "numerus 50 at 5 scudi", Domenico
(if he wanted to bid or stake) would have to throw in only 5 scudi,
not 15.
Passing
The major difference between primero and poker is that a player may
pass without having to put money down. However, the player
must discard one or two cards from his hand, and then draw the
same number from the deck. As all the hands but numerus and maximus
require four specific cards, this usually destroys the hand that you
hold. Note also that you cannot "fold" out of a primero
hand (like you can in poker).
Forced Staking
If no one bids, the hand ends and the player with the highest hand
type wins; if two players have the same hand type, the higher point
total wins the pot. (If no one bids, the pot is likely to have
nothing in it!)
If someone does bid, this bid must be covered. If someone else stakes, the bid is considered covered. However, if play returns to the last person who bid without being covered, the last person to pass must stake; this is called a "forced stake" (my term). The sequence of play is: pass, draw one or two cards, then stake. (The forced stake is one of the reasons why you are warned against playing primero with those who have much more money than you, for obvious reasons)
The last player to bid must then prove to the player who was forced to stake that he has at least what he claimed. Note that in all cases but numerus and maximus, this involves showing the player all four cards. After this, the player who was forced to stake may make a bid without naming hand type or point total. This bid does not have to be covered (but all players who pass must then draw as usual). If play returns to the player who was forced to stake without any additional bids, the hand ends as above.
Since Alberto dealt, it is Bernardo's turn. He passes, so he must discard one or two cards and draw the same number. He discards the ace of hearts and draws the 4 of diamonds, giving him a primero.
Cinthio also passes; he discards the 4 of spades and draws the 3 of diamonds. This gives him a numerus, which is practically worthless.
Domenico already has a primero (one of each suit). As his point total is 47, he can bid up to a primero 47. Understating his bid, he announces a numerus 30. He bids 5 scudi, and keeps his cards.
Now it's Alberto's turn. He passes and does not have to put in money. Hoping for a maximus, he discards the 2d and 5h, and draws Ac and Js.
Bernardo has a primero, so he stakes (throwing in 5 scudi) and bids. His hand total is 61, so he bids numerus 32 and 10 scudi.
Cinthio passes, throwing out the Qd and getting a 6s. A good card, but it still leaves him with a numerus.
Domenico realizes his point total is nearly the lowest possible one for a primero (which he suspects Bernardo has, even though he only bid a numerus), so he discards his two face cards (worth only 10 points apiece), and draws Qh and 3h.
Alberto, though his point total is very high (65) is also pretty sure that Bernardo has more than a numerus. Thus he discards his Ad and Js hoping to get the maximus which would save him...and gets a Ks and 2h instead.
Since no one has covered the bid that Bernardo made (10 scudi), Alberto must cover it. He throws in 10 scudi; Bernardo shows him that he actually has a primero 61. Alberto smiles convincingly and throws in 10 more scudi.
Now Bernardo is in a quandry. He can throw in 10 scudi and keep his hand. Or he can pass, draw one or two cards (probably destroying his hand) and not throw in 10 scudi. He throws in 10 scudi.
Cinthio passes, throwing out the 6s, and drawing 5d. He still has a numerus.
Domenico also passes, throwing out his Qh and drawing the 6d.
Since play has returned to Alberto and no one has bid, the hand ends. Only Bernardo and Domenico have anything higher than a numerus. Since both of them have primero (Bernardo with a Qc As 7h 4d and Domenico with a 5s 2c 3h 6d), the point totals are compared. Bernardo's point total is 10 + 16 + 21 + 14, or 61; Domenico's point total is 15 + 12 + 13 + 18, or 58. Bernardo wins the hand.
Note that if someone had bid after Alberto's forced bid, play would continue normally. For example, suppose Bernardo cackled maniacally, threw down 10 scudi and said "Primero 40!". Then play would continue normally; if no one felt they could beat Bernardo's primero 40, poor Alberto would have to cover the bid again.
Variants
There is circumstantial evidence for certain variants. In no
particular order:
How This Redaction Came About
The English translation of Liber de Ludo Aleae, done by Sydney
Henry Gould, appears as an appendix to Ore's Cardano: The Gambling
Scholar. When I first saw it, and read Ore's (brief) discussion
of primero in the body of the book, it occurred to me that it might be
possible to turn Cardano's brief description of the game into a set of
playable rules. The the page numbers refer to the Dover edition.
p. 206 to 207 discusses the deck, card values, and hand types, which are exactly as in the rules above.
p. 207, bottom: Cardano writes "Also chorus can always be concealed for primero and for fluxus when another has announced it". I have interpreted this as the rule of understating bids.
However, there is another possible interpretation. The previous sentence says "It is not permissible to count diverse bids as more than the greatest of these". On page 206, Cardano writes:
"Now there are two kinds of primero. In one, the greater number wins, and this number is different according to the nature of the hands.."(The other type is where the lesser number wins --- a type of lo-ball poker --- though this is "very little in use".) This and some other isolated quotes suggest that one player announces a hand type, and everyone tries to build that hand type. If you do not have that hand type, you lose, regardless of whether what you hold in your hand is a higher hand type. For example, if the bid is primero, and you have a fluxus --- you lose the hand! As I'm writing this, it occurs to me that this would make for a much more interesting game; it also has the effect of upping the ante of the pot, as players bid to keep the hand from ending. It also has a nice sort of symmetry: a maximus is a numerus, and a chorus is a primero.
p. 208-209: Cardano discusses dividing the pot. I have eliminated this rule and substituted "winner take all", in the interests of ease of play. Also, Cardano is not very clear on who gets which part of the pot.
p. 212 (Cardano spends a few pages discussing cheating): This is the origin of the stake-or-draw rule. Cardano's exact statement is:
"He who, not having announced his primero or fluxus, shall have increased the deposit, except when purposely changing cards, loses his deposit; but if he has not increased it, he is compelled at the will of the others to change his cards..."This sentence is fairly confusing. It seems to relate to the pot splitting mentioned earlier. If one uses a "winner take all" scheme, then (it seems reasonable to me) to interpret the last statement as saying that if one does not increase the pot, one must change cards.
p. 213 discusses the forced stake. Also, the necessity of exchanging one or two cards if one does not stake is explicitly written out.
Some things that are my invention: