Harp players frequently talk about what position they are playing in.  The position specifies the "where/how" of the root note in the scale being played.  By "where" I mean "which hole", and by "how" I mean how the hole is played, i.e. blow, draw, bend, or overbend.  Position is a useful term because diatonic harps come in all keys, but the relative note layout for each key is the same.  This means that once you know a song on a harp in a certain key you can use the same pattern of "where/hows" on any other key and still be playing the same song, or phrase or lick or riff, just in a different key.  The consistency of patterns associated with scale types (e.g. the blues scale) makes the term position useful for communicating with other harp players, though musicians who play other instruments will have no use for the term--it won't tell them anything!

Positions are numbered according to the circle of fifths.  First position is where you start in the circle, and each step clockwise is one position higher.  That is, as you add sharps to the key of the harp you increase the position number by the number of added sharps.  For example, on a key of C harp playing 1st position puts you in the key of C.  To figure out which key is 2nd position, go one step clockwise from C in the circle of fifths (i.e. add one sharp to the key) and you get G.  3rd position is another step clockwise (i.e. 2 sharps added to the 1st position key) which is D, and so on.  Rather than memorizing a table of positions for each harp key it is much preferable to learn the circle of fifths, which is far more useful.

Since there are 12 distinct notes in a chromatic scale and in the circle of fifths, there are 12 different positions on the harp.  When numbering the positions based on going clockwise around the circle of fifths we are essentially talking about "sharp" positions, since each clockwise step gives us a scale with one more sharp than the previous scale.  It is pretty unnatural to think about a key having 12 sharps however!  For this reason, some players talk about "flat" positions as well as regular positions.  The flat positions are named and numbered according to a progression counterclockwise around the circle of fifths.  So first flat on a key of C harp is the key of F.  In terms of the "where/how" of playing, 1st flat is identical to 12th position.  Similarly, 2nd flat is identical to 11th position, etc.

There is a natural mode associated with each position.  By this I mean that by using just the natural non-bent unaltered blow and draw notes on the harmonica, when you start at a different where/how note you are playing a different mode.  Here are the natural modes associated with some common positions.

1. First position is a major scale. The mode name for the major scale is called Ionian.
2. Second position is a major scale variant with a flatted 7th.  The mode is called Mixolydian.
3. Third position is a minor scale.  There is more than one minor scale, and the mode for this one is called Dorian.
4. Fourth position is the natural minor scale whose mode is called Aeolian.
5. Fifth position is another type of minor scale.  The mode for this one is called Phrygian.
6. Sixth position has a sort of major scale feel and is called Locrian.
7. Twelfth position, also called First Flat, has a major scale type feel and is called Lydian.

1st position, also called "straight" harp, is the natural key of the harmonica. In other words, if you have a key of C major tuned harmonica, first position utilizes the scale tones of the C major scale, which starts on the C note.

2nd position, commonly called "cross" harp, starts on the 2 draw and uses a scale a 5th higher than the natural key of the harmonica.  2nd position is the most commonly used harmonica position for Blues, Country, and Rock music because it makes use of draw notes much more than 1st position.  This is important because the draw notes for holes 1-4, and 6 can be bent, allowing all the notes for the blues scale to be played, as well as being able to be played with more variety and nuance. For a key of C harmonica, the 5th scale tone is G (C=1, D=2, E=3, F=4, G=5), so playing 2nd position on a C harp is playing in the key of G (technically the G "mixolidian" mode).

3rd position, sometimes called "slant harp" or "double-crossed", is the scale starting another 5th up from 2nd position, e.g. for a C harmonica, D. (2nd position G=1, A=2, B=3, C=4, D=5.)

To be able to easily determine the music key associated with any key harp in any position, the harp key needed to play in any position for a particular music key, or the defacto position you are using to play a particular key music on a particular key harp, see the section on the Circle of Fifths.