The more notes you add to chords, the more difficult they are to play and the more muddy they can sound. For this reason it is common to omit notes from extended chords, keeping only the notes which most clearly indicate the quality (sound) of the chord. The most frequently omitted note is the 5th, although sometimes the 3rd or the root can be omitted.
Usually the bass will be playing the root note anyway. With a dominant 7th chord, as long as the b7 degree is in the chord you still get the effect of a 7th chord. With a dominant 9th chord, as long as you have the 3rd, b7th and 9th, you have the effect of a 9th chord, etc. This is demonstrated in the following diagram with an E Ninth chord.
Unless you have huge hands, it is very difficult to play this chord on the guitar with the notes in the order they are written according to the formula. All players rearrange and omit notes when playing extended chords. It is good to work out voicings for yourself, as this will improve your theoretical knowledge, your musical knowledge and your ear.
However, through the years certain voicings have become standard because they clearly indicate the chord type and they sound good.
The following diagrams show two commonly used E9 chord shapes which have the 5th omitted. The formula for an E9 chord is shown within each shape, with the position of each degree written next to the fingering.
Play each one first as a whole chord and then as an arpeggio and listen to the effect produced by each degree of the chord. The sound of a 9th chord is clear and unambiguous. There is no necessity for the 5th degree in either voicing. This principle can be applied to all extended chords.
A sixth add9 chord is created by adding a 9th degree to any major 6th chord. For example, a C6/9 chord contains the notes C, E, G, A and D (1, 3, 5, 6 and 9). The following diagrams show some common open sixth add9 chord shapes.