The nation's capital may also be the nation's jazz capital. What other city boasts 21 bands that, in the aggregate, recreate every style of traditional jazz from early New Orleans to Chicago Style to New York Nicksieland to the 1940s San Francisco two-beat of Lu Watters' Yerba Buena Jazz Band? To these idioms add ragtime, represented by the Rosebud Ragtime Ensemble; jug band music, which spontaneously erupts from the Sunshine Skiffle Band; "the hummable melodies of the first half of this century," a specialty of the Band From Tin Pan Alley; and the late 1920s hot dance band charts that the Royal Blue Orchestra trots out.

All 21 bands are packed into a two-disc album, "The Bands of the Potomac River Jazz Club," available for $12 from PRJC Record Sales, 9228 Bailey La., Fairfax, Va. 22031. Or save two bucks and buy it for $10 at this month's PRJC monthly concert, at 8 p.m. Saturday at the Twin Bridges Marriott. On the club's annual New Members' Night, the program will feature two of the very bands heard in the two-record set, Stevie and the Sheiks and Wild Bill Whelan's Dixie Six. (For a weekly update on local traditional jazz action call the nonprofit PRJC's "hotline" at 532-TRAD.)

"I thought that our local bands had reached a point generally where they would stand up to this kind of a presentation," explains Dave Robinson, who compiled the album and plays cornet with several of the bands therein. He collected tapes of each band, selected one number each and wrote the liner notes. "I've sent some copies around the country, and it's been getting very favorable reaction, which is encouraging," he says.

The album is dedicated to the late Beale Riddle, Robinson points out, "because he was so instrumental in bringing musicians together, promoting the music, forming bands and generally just being a friend and a fount of knowledge to all the local musicians." Drummer Riddle, who counted Louis Armstrong, Jack Teagarden, Stuff Smith and many other jazz greats among his close friends, died last March. He had, at one time or another, been the regular timekeeper of at least half a dozen bands in the Washington-Baltimore area and had subbed in many others.

Most of the bands on the album have the instrumentation of traditional jazz groups -- trumpet, trombone, clarinet, piano, bass and drums -- but several include in their lineups instruments not often represented today or even yesteryear, such as the bass saxophone wielded by Jimmy Hamilton of the Manassas Festival Jazzers and the euphonium that Frank Mesich blows on the Buck Creek Jazz Band's "Red Wing." The Rosebud Ragtime Ensemble uses flute and piccolo in "On the Pike," and the Sunshine Skiffle Band throws everything but the kitchen sink into its madcap "Tight Like That" -- including fiddle, jug, washboard, gut-bucket bass and leader Gil Carter's "rhumba box," a footlocker-size case on which the player sits and pounds with the palms.

How can anyone resist a collection of bands with names as colorful as the Fallstaff Five Plus Two, the Pontchartrain Causeway New Orleans Jazz Band and Southern Comfort? But as varied as the music is and as charming their appellations, the award for uniqueness goes to the PRJC Brass Band, which winds up the album with "Bugle Boy March." The 10-member unit organized by Riddle in 1980 "has performed in many of the most ambitious parades in Washington and environs," Robinson writes in his liner notes, "collecting trophies as it goes."

Marching jazz bands. Now that's a tradition that goes all the way back to the beginnings of the -- well, the jazz tradition.