“Mainstream Jazz” is an admittedly boring term that happens to describe some very exciting music. Coined 50 years ago by the British scholar and producer Stanley Dance, mainstream was, in a nutshell, small-band swing. If you took the groups of the big-band era and got rid of most of the band – with one Buck Clayton instead of a whole trumpet section, one Flip Phillips in place of five reeds – then encouraged great soloists to stretch out in front of an equally gifted rhythm section, mainstream jazz was what you got.

At the same time, the modern jazz world was splintering off from bebop to cool jazz and hard bop. Later there would come free jazz, soul jazz, and avant-garde. But already so-called mainstream jazz was in itself heading in many different directions. You can hear this on “Columbia Small Group Swing Sessions 1953-62” (MosaicMD8-228, mosaicrecords.com) and “The Complete Norman Granz Jam Sessions” (Verve B000325202), two boxes of marvelous music made by major jazz musicians of the swing era, but recorded after that era was finished.

The contrast between the music on the Columbia box and the Norman Granz box is striking. The Columbia sessions include a string of extraordinary players – Buck Clayton, Ruby Braff, Illinois Jacquet – who doubled as bandleaders and occasionally composers. They each had a unique, personal vision for what they wanted to bring to their sessions. And they each continually experimented and redefined what they wanted within the larger concept of mainstream – including off-beat tunes and original instrumentation, like Braff’s trumpet-and-vibes sessions and Clayton’s blend of hot horn and swinging organ.

The Verve sessions, by contrast, are dominated by one man: Norman Granz. The original LP jackets had the words “Jam Session” in big letters, with the producers’ name significantly smaller, a relationship reversed on the current box. I am sure Granz himself would approve, since he spent his career advocating the causes of jazz, civil rights, and his own self with equal fervor.

As arranger Billy May once put it, Granz’s only idea of great music was two honking tenors, a rhythm section, and 5,000 screaming fans. He wasn’t interested in anything beyond lengthy blowing sessions in which great soloists would play everything that could be played on the most familiar jazz standards and blues. In this, he anticipated the free jazz movement of the 1960s and 1970s, much (perhaps too much) of which was about extended blowing, to the point that horn players would give entire unaccompanied solo concerts.

The Granz Verve sessions are about reducing jazz to its most elemental essentials. Granz seems to be reducing the concepts of jazz composer and even producer to afterthoughts – the idea was that the guys would just show up and play the first thing that occurred to them. The “Jam Sessions” are a kind of maximal minimalism, in which the producer gives the listener both as much and as little as possible.

The current disc includes five studio dates (as opposed to his live Jazz at the Philharmonic recordings) from the four-year period between 1952 to 1955. The result is a five-disc set containing a total of only 18 tracks, most of which are at least 15 minutes long with minimal ensemble work and lots of improvised choruses.

This is the kind of idea that hurts as much as it helps it, because it doesn’t work unless you had soloists of the caliber of Granz’s: Wardell Grey, Stan Getz, Charlie Shavers, Ben Webster, Benny Carter, even Charlie Parker himself. With the exception of Sonny Rollins and Lee Konitz, there’s almost no one playing today whom I would like to hear stretch out in an endless blowing session like this. Yet for Granz, with these swing and early modern colossi, it worked.

The first jam session is the most famous, since it featured Charlie Parker and represents the only time that Bird recorded with Johnny Hodges and Benny Carter, the other two finest alto saxophonists of his time (and all time).Granz had produced an alto summit for the ages, yet he doesn’t seem to have realized what he had done. So he added soloists on other horns: Charlie Shavers on trumpet, Ben Webster and Flip Phillips on tenor saxophones.

Instead of leaving the three altos to play together or even separately, their solos go streaming by along with everyone else’s on “What Is This Thing Called Love?” the Granz innovation, “The Ballad Medley,” and two blueses – one fast (“Jam Blues”) and one medium slow (“Funky Blues”).Yet the solos are great, and it’s fascinating to hear how Parker, Hodges and Carter react to the same basic blues changes and slow tempo on “Funky Blues.”

Around this time Columbia’s George Avakian was producing a series of protracted jam sessions built around Buck Clayton; these were collected by Mosaic in 1993 (MD6-144). The current Columbia swing box has no such theme, yet all the musicians are out to find something new to do with the classic swing style. And so several fascinating themes emerge.

The first is great swing trumpeters, starting with the veteran Clayton and the relative newcomer Ruby Braff, who, between the two of them, dominate more than half of the sessions here. The other trumpets are Clark Terry, Roy Eldridge, and Sweets Edison. The next horn in the pecking order is the tenor sax, represented by Coleman Hawkins, Ben Webster, Illinois Jacquet, and Buddy Tate.

The Columbia box includes some albums I’ve known all my life, most notably the marvelous “Braff!” (missing, alas, the great cover photo of Ruby in his undershirt), which matches the trumpeter with Hawkins and Ellington trombonist Lawrence Brown. There’s also Jacquet’s wonderful “comeback” album, and producer Mike Berniker’s pairings of Hawkins & Terry (disappointing) and Webster & Edison (better), both from 1962.

The set also contains a fare share of surprises, including an appearance by Marlowe Morris, a rare Hammond organist who was more of a swing mainstreamer than a soul-jazz player a la Jimmy Smith, and a wholly unissued album with Braff and Pee Wee Russell. How I wish the long-lost Johnny Easton-Wayne Shorter session were here as well.

Especially rewarding are two albums by guitarists I would have classified as modernists, co-starring mainstream horns, Kenny Burrell with Jacquet and Herb Ellis with Tate and Eldridge. At the time, Mr. Ellis was holding down a “day job,” so to speak, with the popular Dukes of Dixieland; in a big surprise, the guitarist is joined by his bandmate in the Dukes, trumpeter Frank Assunto. It’s a terrific date, and a great example of a Dixielander and a bebopper meeting in the swinging, mainstream middle.