This question originally appeared on Quora.
Answer by Fred Kanel, I sing baritone with the North Georgia Barbershop Singers and two quartets
The short answer is that baritone is the hardest but most interesting part and singing it in a group of any size is great. Singing it in a quartet is fantastic!
The barbershop parts
The melody is sung by the lead; he's the center of the audience's attention when a quartet is singing. The tenor is above the lead, often singing the kind of harmony that people improvise in casual group singing. The bass sings the low notes, often dramatically low ones at the end of phrases. The baritone sings between the lead and the bass, singing notes that people aren't normally aware of but that are essential. This makes the baritone part sung alone sound strange because it doesn't match either the melody or the normal harmonies that people are used to hearing.
What I thought singing baritone would be like
When I was around barbershoppers at their conventions and tried to do casual group singing, it was evident that baritone was the part my vocal range was best suited for. But I hated it and thought it was a terrible part to have to sing. It wasn't melodic; it jumped around from place to place, often with funny intervals between successive notes; it had lots of accidentals (i.e., sharps, flats, or naturals marked in the music). It seemed to me that the part was made up of the "garbage notes" that somebody had to sing and so were given to the baritone. (I later learned that barbershoppers actually joke that baritones have to sing the notes nobody else wants!)
What singing baritone is actually like
When I finally joined a barbershop chorus and started really learning my part instead of trying to sight-read it on the fly, my outlook changed. Once all those accidentals became second nature I realized that they were what gave the real flavor to the song and made it interesting - like the seasoning in the stew.
Barbershop music frequently uses a special kind of chord, called a Seventh chord, which has a distinctive sound. This chord is very similar to most chords in "normal" music (called tonic chords) except that tonics and sevenths differ by exactly one note. And that note is often sung by the baritone, so his voice is the one that makes it a seventh - without him it's just a tonic.
Once I learned my part to the chorus' repertoire, I really took pleasure in singing a part that not everybody can sing but that is so vital to the barbershop sound.
What singing baritone in a quartet is like
Now I sing baritone with two different quartets. In a chorus, your errors of commission or omission will probably not be noticed; in a quartet there's no hiding place. Not only do you have to know your part completely but you have to be able to blend it in perfect synchrony with the other three voices. (Details can await a separate question.) My quartet practices about seven hours a week to continue to improve in these areas.
Although doing this right adds to the challenge of quartet singing, that extra effort is more than outweighed by our sheer joy at hearing the music we create and seeing its effect on our audiences. Usually that's happiness and delight but on more than one occasion we have watched people cry at particularly moving passages in our songs. The baritone in the quartet has the pleasure of giving the music the nuances of its special sound and the awareness that he is the guy who's doing that.
The joy of singing barbershop
At barbershop conventions before I started singing, I would watch four guys get together in a restaurant or a hotel lobby and open their mouths and create this amazing music out of nothing but their four voices. I couldn't stop grinning from ear to ear and no one else listening could either. Now that I sing in barbershop quartets I actually am one of those four guys who light up peoples lives that way. It's an amazing experience and I can't recommend it highly enough.
You don't have to read music or understand music theory to sing barbershop. Barbershoppers are really welcoming of newcomers, and they have CD's of your part being sung that you can learn from (which is the method I prefer.) If you've ever wanted to sing barbershop music - perhaps in a group so you don't stick out while you're learning - go to http://barbershop.org and find a chapter in your area.