If you analyze the notes of scale tone 7th chords, you will notice that they are all based on the C major scale tone triads and each one has another 3rd interval added above it. If the left hand is playing the root of each chord, it is possible to leave the root note out of the right hand voicing. A common method of playing 7th chords it to use only triads with the right hand and play the root note with the left hand. If you choose the triad two notes ahead in the scale of the key you are working in, you can imply a 7th chord. This is called chord substitution. For example, a CMaj7 chord, play a C bass note with the left hand and an Em triad instead of a CMaj7 with the right hand. The combined notes are C, E,G, B, which is a CMaj7 chord. This could also be described as Em/C . A comparison is shown here.
This example shows how substituting the scale tone triad two ahead can be applied to the C major scale. The same principle applies to all keys.
Most slash chords add up to another chord type (e.g., some kind of 7th chord). Sometimes it makes more sense to describe the chord as a slash chord, (e.g., when a pedal tone is being used) and other times it makes more sense to describe the chord as what the notes add up to. The following example demonstrates both a slash chord (F/C) and a minor 7th chord created by substituting an Eb triad for a Cm7 in the right hand part.