Music theory people say that chords with three or more notes can appear
in two forms, closed or open. In closed form, all the notes are
very close together, while in open form, none of the notes are close
together. In either form, the chords still contain the correct
three or more notes. Either form can be in root position
(root of the chord as bottom note), first inversion (second note of the
chord on the bottom) or second inversion (third note of the chord as
lowest note.) And, with four-note chords, either closed or open
form can also appear in third inversion (fourth note of the chord on
the bottom.)
With closed form chords, as you scan the chord from low to high,
only adjacent chord tones are used to form the chord. For
example, a D minor chord contains notes D, F and A.
In root position, the b ottom note is a D. Next higher possible
chord tone is an F. In closed form, we include that F as second
note in the chord. Next higher chord tone is an A. In
closed form, we include that A as third note in the chord.
With open form chords, when building the chord from low to high,
one chord tone is always skipped when determining all but the bottom
note of the chord.. For example, another D minor chord, still
requiring notes D, F and A. Still in root position, the bottom
note is a D. Moving upwards, the next chord tone would be an
F. But in open form, we skip that note. The next chord tonw
would be an A. In open form, since we already skipped the F, we
use that A as second note in your chord. Again, moving up, the
next chord tone would be another D. But again, in open form, we
skip that note. Moving upwards again, the next chord note would
be an F. In open form, since we already skipped the D, we use
that F as the third chord note.