A chord is a collection of notes which all start at the same offset, i.e. they all start at the exact same time. In Alda, a chord is expressed as notes with slashes in between them: c/e/g

It's acceptable to have octave changes in between the notes of a chord, which allows for chords spanning multiple octaves: c/g/>c/e/g

The notes in a chord can all be different lengths, in which case, the next note event after the chord will happen after the shortest note in the chord. This makes it easy to have chords with shifting tones, e.g.: c1~1/>c/<e4 f g f e1 (also, note that, just like with sequential notes, each note duration becomes the default for all notes that follow - both C notes in this chord are 2 whole notes long).

Alda also allows you to use rests in a chord. Because the next note event after a chord will start after the shortest note/rest in the chord, this can be useful for writing melodies entwined with chords, e.g. c1/e/g/r4 b e g