Many times I have heard or read that Cajuns play their accordions up a fifth. One web site says that Cajuns play up a fifth and go to great lengths to avoid playing the seventh note, Ti, because it will be flat.
I have often puzzled over these statements. I play a very few Cajun numbers, and don't play them up a fifth. On the other hand, I play a few non-Cajun numbers up a fifth because I like the musical effect of the diminished seventh.
Recently I acquired a book of Cajun dance music, "Ye Laille Chere" by Raymond E. Francois. It's a great book, and I got my copy from Amazon with free shipping! (No, they're not paying me to say that. A well-known music dealer wanted to charge $10 shipping plus a three percent surcharge for credit card orders.)
Francois put a great amount of Cajun lore in his book, including the explanation of playing up a fifth.
It seems the Cajuns are true folk musicians. They mostly play by ear after hearing tunes played by other artists. This is the way the tunes are handed down from generation to generation.
The Cajuns have traditionally been lacking in formal education. Instead of taking classes, they just figure out how to play their instruments, like in the old song, doing what comes naturally.
Cajuns played fiddles long before German immigrants introduced them to the one-row German melodeon, or diatonic accordion. They had to make this new instrument play along with their fiddles.
The fiddlers did not like to change their fingering patterns from one song to the next. A fiddle, or violin, (depending on how you fiddle around with it) is easiest to play in the keys of D and A. The fiddle is a G-D-A-E instrument, those letters being the pitches of the open strings. For playing in C or G, the Cajun fiddlers would tune their instruments down one step, making the open strings A-C-G-D. According to Francois, some artists would keep two fiddles so they wouldn't have to retune between numbers.
A C accordion, when played up a fifth, plays in G with a flat seventh note, giving it a sort of bluesey sound. Likewise a D accordion played up a fifth will be in A with a flat seventh.
Now to fill in the blanks. Look at a piano keyboard. Notice the white keys and black keys. The piano is a C instrument. If you go up and down the white keys you play the C Major scale.
Notice that most of the white keys have black keys between them. From one key to the next is a half step. So when you go from one white key to the next when there is a black key in between, you are playing a full step.
Notice also that in each octave there are two pairs of white keys that do not have black keys between them. The keys in these pairs are a half step apart.
The locations of the half steps in a scale will has a tremendous effect on the way the music sounds. More on that later.
On the piano keyboard, when played in the key of C, the half steps are there automatically. The keyboard artist need not even think about them. But when playing in another key, using the black keys, the keyboard player has to keep track of where the half steps are.
There are no automatic half steps on the violin. The fiddler has to put them in by positioning his or her fingers in certain ways.
On the violin, when playing full steps, the fingers are spaced a certain distance apart when they come down on the strings. When playing a half step, one finger comes down smack dab against another, like they are pinching.
When playing a scale beginning on an open string. the middle and ring fingers (second and third in violin parlance) are pinched together for the first octave, which is usually played on two strings. Moving up to the next octave the index and middle fingers (first and second) are pinched together.
Now if you want to play in C on a violin, you start on the G string with the second and third fingers pinched, and finish the octave on D and A strings with the first and second fingers pinched. As you move around on this instrument and play in different keys, the fingering pattern changes. It's easy to see why people lacking in formal music education would find some shortcuts.
Now back to playing up a fifth. Looking at the piano keyboard, if you assign Do to a position other than a C key, you change the location of the half steps in the scale. Move Do up one full step and the scale is in D minor, with the third and seventh diminished or flattened. Move up a fifth, and assign Do to G, and the scale will be G with a diminished seventh, sort of halfway in a minor key.
On a fully chromatic instrument, like a piano or violin, you can play in any major, minor or diminished seventh key you like, as long as you can keep track of where you are playing. (I never could get the hang of this on a piano keyboard, but it's not hard on a violin. What is hard on a violin is making it sound good.) On a diatonic button accordion, playing in the minor and diminished seventh keys is mainly a matter of remembering which way to move the bellows on each note. Once you memorize a song in one of these keys, the bellows direction will just be a part of the song.
A fourth or a fifth will always include one half-step. Looking at a keyboard it might appear that F to B would be a fourth, but it's not. F to B-flat is a fourth.
The fourth and fifth have a special relationship to the musical scale. Pitch is determined by the number of vibrations per second imparted to the atmosphere, which carries the sound. These vibrations are picked up from a string, reed or other vibrating body.
The concert A pitch is 440 vibrations per second. The A an octave up is 880 vibrations per second, and an octave down it's 220 vibrations per second. A fifth splits the difference between octaves in half. Thus the note E would be 660 vibrations or a multiple. The fourth is the other half of the octave.
One of the first things a violin student learns is to recognize the sound of a fifth when two open strings are played together. There's something special about it that the ear and brain can recognize quite easily.
I said above that the violin strings are a fifth apart. That also applies to the viola and cello. The string bass is tuned in fourths.
Fourths are sometimes called upside-down or inverted fifths. Although diatonic accordion rows are usually a fourth apart, by some nomenclature they are called fifth instruments. Concertinas may be an actual fifth apart. This makes them maddening for an accordionist to play because the customary cross-row combinations are not there.