Musical intervals can be likened to measurements that we learn early in life like the inch, the foot, the yard and so on.  Music notes are are also given names for the space that exists between two points.  In the most basic way, intervals can be defined as the distance between two musical notes.  Musicians are often encouraged to know intervals by recognizing them on staff paper and most importantly to recognize their aural effect.  Each interval is a unique distance and thus will give the listener a unique experience with each interval that is heard.  Intervals can  be classified as simple intervals or compound intervals.  Simple intervals are the most commonly talked about intervals in theory while simple and compound intervals are used frequently in music.  There are 13 simple intervals starting with the perfect unison.  A perfect unison actually represents no distance at all.  It is the same note played together or one after another.  Intervals played at the same time are referred to as harmonic intervals while intervals played in succession are called melodic intervals.  The next interval would be the minor 2nd.  A minor 2nd is the smallest measured interval in music using the equal tempered tuning system.  This equal tempered system is used pre-dominantly in modern American music like Pop music, Jazz, Blues, Rock, Country and pretty much all other styles heard in this country.  On a guitar, a minor 2nd would be the distance from one fret to the next fret.  On a piano the distance would be rom one key to the very next key.  Playing harmonic minor seconds will usually be described by the listener as a non-pleasing or dissonant sound.  Playing a harmonic minor second on the guitar can be a difficult task because you’ll have to stretch your fingers pretty far.  Playing a harmonic minor 2nd on the piano is much simpler.  Just play two adjacent notes and you’ll have the sound of the minor 2nd.  Examples of minor seconds are C-Db, C#-D, D-Eb, D#-E, and so on.  The next interval in terms of size would be the Major 2nd, followed by the minor 3rd, Major 3rd, Perfect 4th, Tritone, Perfect 5th, minor 6th, Major 6th, minor 7th, Major 7th, and Perfect Octave.  To learn more about intervals and their names so that you can effectively use them while practicing and playing music, you’ll need to get some reference material.  Click here to see an awesome, yet inexpensive book on music theory for guitar.