It sounds like they mean the interval of a second.
For example, if you have a E major chord, an 'F#' note is a second (major second) above the E.
If that 'F#' is part of a dominant chord we would call the 'F#' a ninth.
These two labels may seems inconsistent, but think of it like this:
- when adding a second to a simple triad we just take one step up from the root,
- when adding to a dominant seventh chord we step up from the root in thirds and place a 9th above a 7th,
In both cases we have a chord root of
E and an added
F# but we count up from the root differently to get either the added
There are some other subtleties worth noting about these chords.
The adding a second and omitting the third can be labeled
sus2 where the second is "suspended" and traditionally would be resolved by moving up to the third. In a very traditional approach this chord could be a tonic, an example in D major would be
A Dsus2 D the high
E over the
D chord resolves up to
x02220 xx0230 xx0232.
If we merely add the second and also leave the third in the chord it gets labeled
9 chord is a type of dominant chord. In order to make that dominant function clear - and to distinguish the ninth from an added or suspended second - we need to have the seventh also present in the chord. For
E9 we need
D natural along with the
...avoid “seconds” when you write music sheets for a guitar...
Personally, I have never seen this advice, but certainly one should try to avoid writing unplayable music.
I tried to provide some specific guitar fingerings to show these various chord types can be played. But, I had to choose specific octaves and positions to get easy to play examples. It could be difficult to play some chords with added seconds/ninths depending on the specifics. For example, adding a second one step above the low root of a barre chord would practically unplayable.
Take care for the playability of the chords you choose. A chord dictionary may be a helpful aid in this regard.