Each fret is a semitone. The instruments are tuned so that the next highest string sounds the same as the one below fretted at the 7th fret. (This means that they are tuned in 5ths.)
The notes from the nut to the 12th fret on a string is called an octave and has 12 semitones and 8 notes (since a note can be an interval of 1 or 2 semitones in a set pattern called a mode - hence Octave). The octave note is the same note as the open string but a whole set of notes higher in pitch. The length of the string determines the pitch (frequency) of the basic note (open string). A longer string gives a slower frequency, and hence a lower note. Holding a string down at a certain fret shortens the string and makes it sound higher in pitch. The frets are positioned at just the right interval to ensure that the notes sound correctly and have the right relationship with each other. (This is called the Well Tempered Scale - not all instruments are well tempered -in more ways than one!)
The mandola family can be tuned easily by getting the bottom string or course in tune and then tuning each higher course to the previous string at the 7th fret - they should sound the same. You can get the bottom course note in tune from a piano, another musician, a tuning fork or an electronic tuner. (See Tuning)
Alto tuning a mandola is to tune a fifth below the mandolin.
The standard mandolin tuning is GDAE (thickest course to thinnest course). What is a 5th below? Well there are only the notes A to G so think of them as a circle. Counting down off A brings you back to G. So take a mandolin note (say E) and count back 5 (E - D - C - B - A). The first string on a tenor mandola (and viola, tenor banjo etc) is tuned to A. The rest of the tuning is CGDA.
Middle C on an alto mandola occurs at 3rd string, 5th fret.
Octave tuning a mandola is to tune the mandola a whole octave below the standard mandolin tuning. This requires that the mandola has a slightly longer scale length (nut to bridge) than the alto mandola and / or a change in the string weights (as the tension is less).
Octave tuning is a favourite tuning in folk music.
Middle C on an octave mandola occurs at 2nd string, 3rd fret.