Justin Slaten and Ryan Fernandez may not be household names yet, but this past season, both rookie relievers were outstanding for the Red Sox and Cardinals, respectively. And they have more in common than just being relief pitchers who had strong rookie campaigns: They were each selected in the 2023 Rule 5 Draft.

Though it has changed considerably over the years, the Rule 5 Draft dates back a century and a quarter, making it older than the World Series. Along the way, it has produced many big league contributors and a handful of stars, but it remains a relatively minor source of talent.

Because the Rule 5 Draft is a somewhat complicated affair, here is a look at how it works, as well as how it has evolved, heading into the 2024 edition, scheduled for Wednesday in Dallas, Texas.

Rule 5 Draft rules

Any club with an open spot on its 40-man roster is eligible to make a selection. Teams pick in reverse order of the previous season's standings.

In 2023, 10 players were picked, eight of whom wound up seeing MLB playing time in 2024. A couple of those selections were especially savvy. Slaten, who was actually taken in the Rule 5 Draft by the Mets but then traded to the Red Sox, proved to be a key part of Boston's bullpen last season, posting a 2.93 ERA over 55 1/3 innings. Fernandez, who was actually selected from the Red Sox in last year's Rule 5 Draft, finished his debut season with a 3.51 ERA over 66 2/3 frames for St. Louis.

The draft also features a Minor League phase. Teams made 63 Minor League selections during the 2023 event.

Players on an organization's 40-man roster are protected from the Rule 5 Draft. To be eligible for selection, a player must either have spent four seasons in professional baseball after signing at age 19 or older, or spent five seasons in pro ball after signing at 18 or younger (as of June 5 of that year).

A team that makes a selection pays a player's previous team $100,000 and places the player on its 40-man roster, then must keep the player on the 25-man roster or injured list for the whole season. (He must be active for at least 90 days, so teams can't hide him on the injured list the whole time.) If the acquiring team removes the player from the big league roster, it must offer him back to the former team for $50,000.

The early years

The event now known as the Rule 5 Draft dates all the way back to 1892, 11 years before the American League and National League held the inaugural World Series. Most commonly referred to as the "Selection of Players" at first, it allowed big league teams to draft players between Oct. 1 and Feb. 1 each offseason, in no predetermined order.

Before the rules recognized affiliated farm systems in 1931, big league teams made their picks solely from Minor League teams, all of which operated in independent leagues. Initially, a pick cost either $500 or $1,000, depending on the level of the player's league. Both the acquisition costs and the Draft period changed numerous times over the next century, with the term "Rule 5" first appearing no later than in the 1941 edition of the Major League rules.

Notable example:

Center fielder Hack Wilson debuted briefly for the New York Giants in 1923, had a strong follow-up in '24, then struggled the next year and was dealt to Toledo of the American Association that August. After Wilson performed well for Toledo, the Cubs acquired him via the Rule 5 Draft coming off a last-place finish in the NL. Over the next five seasons, Wilson hit .331/.419/.612 with averages of 35 home runs and 142 RBIs, including a still-standing record of 191 RBIs in 1930. The Veterans Committee selected him for the Hall of Fame in '79.

The bonus rule era

At various times between 1947 and the start of the annual amateur Draft in '65, baseball had different versions of a bonus rule that was intended to limit signing bonuses and prevent wealthy teams from hoarding talent. Players who received bonuses over a certain amount had to be kept on the big league roster for a certain length of time in order to be protected.

The version of the rule in effect from 1953-57 set the bonus threshold at $6,000 and required an initial two-year stay in the Majors. If unprotected, these players were unrestricted in the Rule 5 Draft, meaning they didn't count against the limits placed on the number of picks a single team could lose in a year.

Notable example:

In February 1954, the Dodgers signed a 19-year-old outfielder named Roberto Clemente out of Puerto Rico, giving him a $10,000 bonus. But instead of bringing him to Brooklyn, the organization sent him to its Triple-A affiliate in Montreal. Clemente got only 155 plate appearances that season and posted just a .657 OPS with two home runs, but that didn't stop Branch Rickey and the Pirates from grabbing him in the Rule 5 Draft that November. It took some seasoning in the Majors for Clemente to hit his stride, but eventually he became one of the greatest outfielders of all time.

Modern times

Between 1965 and 2005, baseball introduced the amateur Draft and free agency, and gradually expanded to 30 teams. The Rule 5 Draft also was tweaked during that time, with the selection price increasing to $50,000, and the time before a player could become eligible rising to three or four years, depending on age.

Some of the best picks from this era didn't actually help the teams that drafted them. For example, the Cardinals took Cecil Cooper from the Red Sox in 1970 but then offered him back, the White Sox took Bobby Bonilla from the Pirates in 1985 but traded him back to Pittsburgh the following July, the Marlins took Johan Santana from the Astros in 1999 but immediately traded him to the Twins, and the Orioles took Jose Bautista from the Pirates in 2003 but put him on waivers six months later.

Notable example:

Outfielder Shane Victorino had the distinction of being a Rule 5 Draft pick twice. First, the Padres grabbed him from the Dodgers in 2002, but a 22-year-old Victorino hit .151 in 36 games in '03 and was returned near the end of May. L.A. never brought Victorino to the Majors and left him unprotected again after the '04 season. This time the Phillies jumped, though they too offered him back to the Dodgers before the '05 season. Fortunately for Philadelphia, L.A. declined, and Philly was able to send Victorino to Triple-A. He returned in September and established himself as a key part of the team's lineup from '06-12.

The current system

In 2006, the pool of players eligible for the Rule 5 Draft shrunk, per the terms of a new CBA. Unprotected players now must be four or five seasons removed from signing, depending on whether they did so before turning 19.

Since then, relatively few players have stuck with a drafting team and made an impact, though Josh Hamilton, R.A. Dickey, Darren O'Day, Brad Keller, Ender Inciarte and Mark Canha are among others who have been selected during that time. Success stories include relievers Joakim Soria (Padres to Royals, 2006), Hector Rondon (Cleveland to Cubs, 2012), Ryan Pressly (Red Sox to Twins, 2012), Anthony Santander (Cleveland to Orioles, 2016) and Garrett Whitlock (Yankees to Red Sox, 2020).

• Here are the 10 best Rule 5 Draft picks of the past decade

There is also a rare gem that appears in the Minor League phase. In 2015, the Cardinals snatched pitcher John Brebbia from the D-backs with the 24th pick, after the righty had signed with Arizona out of the independent American Association. After spending 2016 in the Minors, Brebbia was a solid member of the St. Louis bullpen over the next three seasons before missing 2020 due to injury, getting non-tendered and returning with the Giants in '21. Left-hander Tyler Gilbert also threw a no-hitter in his first MLB start for the D-backs in '21, after being selected from the Dodgers in the previous Minor League phase.

Notable example:

Marwin Gonzalez reached the Majors with the Cubs but got a break when the Red Sox selected him in the 2011 Rule 5 Draft, then traded him to the Astros. Houston had just entered a painful and protracted rebuilding phase, but that gave Gonzalez the chance to make his MLB debut in '12 and gradually develop into a highly valuable utility man. By the time he left as a free agent after the '18 season, Gonzalez had played nearly 800 games for Houston, appeared at every position other than pitcher and catcher, and helped win a World Series title in '17.