Tomorrow at about 5:30 p.m. Central European Time (12:30 p.m. EDT), 115 cardinals will gather in the Sistine Chapel and begin the process of selecting a new pope. But how exactly does that process work? Here are the basics.

Who gets to vote?

Cardinals, but only a subset. Cardinals who turned 80 before the incumbent pope died or resigned are ineligible to vote under rules originally set forth by Pope Paul VI (who reigned from 1963 to 1978). As Pope Benedict XVI resigned Feb. 28, a few cardinals slipped in under the wire, including Walter Kasper (who turned 80 last Tuesday) and Severino Poletto (who will turn 80 next Monday). In total, 115 cardinals will participate in the conclave: 117 were eligible, but two chose not to attend. Indonesian Cardinal Julius Riyadi Darmaatmadja declined to attend due to deteriorating eyesight, which would make reading materials involved in the election difficult. Cardinal Keith O'Brien opted out after he resigned as archbishop of Scotland following allegations that he had made improper sexual advances toward three younger priests and a seminarian.

What's the make-up of the electorate?

Of the 115 cardinal electors, a majority — 60 — represent Europe, with 28 from Italy alone:
The oldest cardinal in attendance will be Kasper, who is 80, and the youngest will be Baselios Cleemis of Kerala, India, who is 53. The median and mean ages are both 72. Every cardinal in attendance was appointed either by Benedict or John Paul II, with 87 appointed by the former and 28 by the latter (this previously and erroneously said 48; we apologize for the error). That means that over three-fourths of the College of Cardinals was appointed by Benedict.

How does voting work?

The short version is that as soon as a candidate has two-thirds of the cardinals' support and accepts, the die is cast and he's the pope. John Paul II changed the rules in 1996 to make a simple majority all that's required for papal election (but only after 30 rounds of unsuccessful balloting), but Benedict XVI changed it back. Then again, over two-thirds of the College of Cardinals was appointed by Benedict, and if they hold similar views, that could mean less gridlock.

The longer version is that the cardinals sequester themselves in the Sistine Chapel and take a series of votes until the process is complete. They are expected to take one vote tomorrow evening, and four votes each day after that, two in the morning and two in the afternoon. The ballots are burned after the two morning votes and the two afternoon votes at noon and 7 p.m., respectively, to prevent post-election analysis of cardinals' selections. If a pope is elected in the first vote of the morning or of the afternoon, that vote's ballots will be burned earlier, at 10:30 a.m. or 5:30 p.m., respectively. Giovanni Battista Re, as the oldest cardinal under 80, will preside.

As the ballots are burned, a second stove is used to emit either black smoke (signifying that no pope has been chosen yet) or white smoke (signifying that there is a new pope). Cellphones are banned and plastic sheeting put up around the chapel to prevent onlookers from viewing the proceedings. Jammers are also used to prevent any access to cell or data networks.

Who's in the running to be pope?

Any baptized Catholic male, theoretically. You don't even have to be a priest. But ever since Pope Urban VI died in 1389, popes have been chosen from among the cardinalate. That doesn't mean, however, that the pope must be a voting members of the College of Cardinals. Indeed, Francis Arinze of Nigeria is thought to be in the running but, having turned 80 before Benedict's resignation, is ineligible to vote. So who has the best shot? John Allen of the National Catholic Reporter, probably the best Vatican journalist writing in English, has a good rundown of the likely possibilities (known in the biz as "papabile") here. Various betting sites, including Paddy Power, Ladbrokes  and Betfair, have odds on the various contenders. Here's where things stand at the moment for candidates who are in the top 10 on at least one of those sites*.
Note: the odds have been updated to correct a calculation error in the initial table; we regret the error. The heavy favorite is Angelo Scola, the 71-year-old archbishop of Milan. He would be a culturally conservative pick, having written a whole book lambasting feminism and, according to reviewer Luke Timothy Johnson, associating homosexuality with "nihilism and narcissism." The argument, Johnson surmises, is that "feminism is responsible for homosexuality, because the more women act like men, the more men are likely to want to have sex with other men." Given that Tarcisio Bertone, Gianfranco Ravasi and Angelo Bagnasco also have pretty good odds behind them, so the markets are giving a clear indication they think the next pope will be Italian. We'll know if they're right soon enough.

*Methodology note: First, I converted each to decimal odds (wherein odds of 2.5 mean I get $2.50 if a $1 pays off), then averaged them (and for Betfair, average the put and lay costs), and then calculated the probability of each papabili implied by those odds (using the formula that the probability = 100/(decimal_odds)%).

Update: The chart has been updated yet again, this time to correct for the house's margin included in the face odds offered by the betting sites. The current numbers should better reflect the papabile's odds.