A tuplet is a group of notes that form an irregular subdivision of the beat. In an 8th note triplet, for example, 3 8th notes (which normally take 1½ beats in a 4/4 meter) are played in the time normally occupied by 2 8th notes (only 1 beat). As a result, each note of the tuplet is slightly shorter so that the notes take the same amount of time to play.
There are many types of tuplets (duplet, triplet, sextuplet, quintuplet, etc.) that can be added to your score. For example, a 16th quintuplet is a group of 5 16th notes played in the time that is usually taken up by only 4 16th notes. Another description of this tuplet is "5 against 4" - 5 represents the "irregular" divisions of the beat and the 4 represents the "regular" divisions. The 16th note specifies the size of each regular division.
Notice how the ruler changes to display divisions for the quintuplet. The ruler always displays the correct subdivisions of the beat for the type of tuplet you are entering.
You do not have to confine yourself to the "regular" notes that define the duration of the tuplet. You can place any note in the tuplet, as long as it fits within the total duration of the tuplet. Contrast the quintuplets above with those in the following example:
Depending on the tuplet and the note type, there are specific tuplet types that can be created. For example, in the time that 2 8th notes normally take, you can create a triplet that consists of 3 8th notes, a quintuplet that consists of 5 16th notes, a nonuplet that consists of 9 32nd notes, and so on. Some tuplets are neverUsed in certain meters. For example, you cannot have a half-note triplet in a 3/4 bar since the triplet spans 4 beats and there are only 3 beats in the measure.