But there's also the unexpected, such as an occasional non-bluegrass song presented in a harmony-rich setting.
That's why it made perfect sense that lead vocalist Russell Moore had been recording "Milk Cow Blues" at Doobie Shea studios in Boone's Mill, Va., just before an interview.
And doing it as a solo vocal at that.
"It's a little stretch for us and maybe for everybody else," Moore says, "but it's a fun song to do, and being from Texas I heard a lot of swing-influenced stuff."
"Even with our first recording we a song called Miles and Miles of Texas,' which is swing-influenced, and we did Steel Guitar Rag,' which has the same sort of feel. We like to throw one in every now and then."
The Georgia-based quintet recently released "Live At The Mac," its third album for Rounder Records and sixth overall since forming in 1991, and is polishing material for a new album due early next year.
Dobroist extraordinaire Rob Ickes of Blue Highwayplays on seven tracks. Moore calls Ickes "one of our good friends and one of the best players on the circuit right now."
As "Live At The Mac" attests, the group doesn't need special guests to sound special. Recorded live at The Mountain Arts Center in Prestonsburg, Ky., last May, IIIrd Tyme Out delivers complex but satin-smooth vocal arrangements and hot but tasteful playing on demand.
"That was a special show for us," Moore says. "The people were definitely up for it."
As the group hits the bluegrass festival circuit for the summer, the 14 tracks give a good cross-section of the IIIrd Tyme Out sound.
Tradition gets its due with three Bill Monroe numbers - including the opener, a fast and furious "White House Blues" - as well as songs associated with Flatt & Scruggs and Jim & Jesse McReynolds.
Having begun with a classic, the album fittingly concludes with a modern composition, Ronnie Bowman and Tim Massey's "I'm Better Off Without You." There's also Carl Jackson's "Erase the Miles" and Cathy Gosdin's "Till the End," a 1977 country hit for Vern Gosdin.
Gospel harmonies shine on "He Said If I Be Lifted Up" and "It's a Lonesome Road." The latter showcases stellar bass vocalist Ray Deaton while underscoring IIIrd Tyme Out's vocal dexterity and flexibility.
"Ray actually sings the lead through the verses, a bass lead, and at the chorus Wayne (Benson, vocalist and mandolin player) takes over the lead and we build the quartet around him," Moore says.
Aside from Moore's soulful lead vocals, Deaton is the band's most recognizable voice. "He'll sing bass parts, he'll sing harmony parts, he'll sing lead or he'll sing tenor," Moore says. "He does have quite a range."
The group's multi-layered vocals, such as on "Everybody's Gonna Have a Wonderful Time Up There," a Southern gospel-style number on "Living On The Other Side," can boil down to 1:56 on an album - after days of work.
"A friend of ours from up in Kentucky sent that song to us and said, Boys, I believe you could just tear this one up,'" Moore says, "We wanted to do something that switched out the lead parts and gave Ray a little shine."
For those used to the busy recordings that mark - some say mar - modern country, pop and rock, IIIrd Tyme Out takes you back to basics while dazzling with jaw-dropping vocals and subtle instrumental interplay. Steve Dilling's banjo and Mike Hartgrove's fiddle are more than mere icing on the cake.
Non-bluegrass fans also find familiar numbers such as "Tennessee Waltz" on "Live At The Mac" or their signature a cappella version of the Platter's "Only You (And You Alone)" on the 1995 Rounder album, "Letter to Home." Moore also acknowledges the impact of Alison Krauss' cross-over country success with "When You Say Nothing At All" on the Keith Whitley tribute album.
"She was booked on several bluegrass festivals and would draw in the country audience," he says, "We were fortunate enough to be able to perform either right in front of their show or a couple of spots in front of them. People were there to see Alison, but they would listen to our show, too.
"They would come up after our show, and say, You know, this is great, we didn't know that was what bluegrass was all about," he says. "Maybe they didn't care for some of the older stuff that was recorded years ago."
The new fans, Moore recalls with a chuckle, also bought "everything we had."
"Not just albums, but T-shirts and hats - anything you've got, they want to take it home with them. It's a little bit different audience. They're younger people and maybe they're not burdened down as much as the rest of us are with a lot of bills," he says. "They get some money in their pocket and it just burns a hole in it!"
Moore modestly gives thanks when told his composition "Daydreams," featured on "Letter To Home," would be a great tune for a country or pop act to record. "We get a lot of requests for ballads, including that one," he says, noting that the next IIIrd Tyme Out album will have ballads - and much more.
"We're trying to make this recording with a lot of variety. We don't want to be complacent, always go down the same path," he says. "It gets boring for everybody.
"Now, your hard-core traditionalist may not find us as favorable as another group, but people who do come listen to us know they can expect a cappella, gospel, bluegrass, the ballads - everything rolled into one."
Aside from staying fresh, Moore says, longevity is a goal. The band has had the same lineup for about five years, he notes. "We enjoy each other's company. It makes road life a whole lot easier."
Awards also make it easier for IIIrd Tyme Out to keep going, but Moore says they feel no pressure to win.
"We're fortunate to have bands that think as much of us as they do," Moore says. "We feel like we have a really good rapport with people. We're not trying to toot our own horn or anything - we just want to play some music."