What do we learn in this chapter?
A triplet splits a duration into three equal parts. The total duration of a triplet is usually equal to the duration of two of its note-type. For example, the total duration of the eighth-note triplet is equal to two eighth notes. Likewise, a quarter-note triplet equals the duration of a half note or two quarter notes.
Have you seen triplet-like groups in 6/8 meter?
It is actually not a triplet. It is three regular eighth notes having the total value of a dotted quarter note and written without the number 3. A duplet or quadruplet, instead, could be employed in a compound meter. Eighth duplets/quadruplets in 6/8 equal to the total duration of a dotted quarter note.