It is important that your instrument is in tune when you play. There are two aspects to this. The first is that it is in tune with other instruments playing at the same time and the second is that it is in tune with itself
There are again two aspects to this. It depends on whether all the other instruments are at concert pitch (or can be tuned) or not. Reed instruments such as melodeons and concertinas cannot be tuned (without major surgery) but stringed instruments and even pipes and whistles to some extent can be tuned.
There is always the option to tune to other instruments anyway if you do not have a tuner. i.e. you rely on someone else having a tuner.
This applies to fiddles, guitars etc too but each instrument differs in the relationship on one string to the next. This advice is for instruments tuned to the standard GDAE for mandolins and their like which are tuned in what are known as perfect fifths. This is an interval of notes spanning five diatonic scale degrees. That is 5 notes in the scale of the open note which for each string is G, D, A or E. Starting with G, the lowest string open note, a scale of G goes G-A-B-C-D (yes and E-F# and back to G). The first five notes interval is G to D, so the next string is tuned to D. Now a scale of D goes D-E-F#-G-A etc so the next string is A and finally the scale of A goes A-B-C#-D-E etc so the top string is E.Now do not be confused, but these 5 notes of scale happen to be on a chromatic fretboard (chromatic means that every note of every key is possible) which is laid out as a semitone per fret. That means that taking the lowest string (G) again and going up the fretboard from open, you actually have G-Ab-A-Bb-B-C-Db-D. The notes Ab (or G#) Bb and Dd (or C#) aren't in the scale of G but it does mean that the D is at the seveth fret. If you want to know more about the theory behind this, see The Ionian Mode which gives us the modern major scale. Basically, the a major scale from any note is Whole Tone, Whole Tone, Semitone, Whole Tone etc where a whole tone is 2 semitones, or 2 frets.
What this means is that each string, when fretted at the seventh fret and played, sounds the same as the next highest string. This is how you tune a GDAE instrument to itself using one string which has been tuned to a reference (another instrument or a tuning fork). If you are playing alone, getting the 4 strings or courses tuned to each other without a reference is fine, but the whole tuning may get progressivley sharper or flatter than concert pitch. A small variation is not a problem and you can play perfectly well, but a large variation means that the tension becomes noticeably slacker or tauter, affecting the neck of the instrument, the action (height above the frets) and your playing.
With an electronic tuner you can tune each string or course using the indicator on the tuner. This is best done in a quiet envionment unless you have a pick-up on the instrument or a clip on mic which you can plug into the tuner. Some allow you to vary a reference away from concert pitch but don't do it unless you know what you're doing! It is better to tune to another non-concert instrument and then to itself (see above)
|Digital Tuner (Analogue Display)||Analogue Tuner|
|These tuners have built-in microphones to pick up the sound of the instrument so can only be used easily in quiet environments. They also have an input and output socket so they can be used with electric instruments in-line.|
|Clip-on with LED bar indicator||Clip-on Digital with Analogue Display|
|These tuners have contact microphones built-in and can be clipped to the headstock. They are much better at rejecting noise in a session and some musicians leave them in place all the time.|
|Digital Tuner using an external contact microphone in the input socket, clipped to an acoustic instrument|