On a stringed instrument, A Tremolo is a rapid back and forth movement of the bow on the same note. On other instruments, A Tremolo is a trembling effect between two notes or chords in alternation, not to be confused with a trill.
A roll is a way to sustain sound on a percussion instrument.
A slash mark tells the player to improvise their own rhythm and melody to the chord symbols above the staff.
A grace note is a note that quickly goes into another note. For example, if you saw the note D with a grace note of C behind it, you'd start in C and very quickly move onto and play the D note
An appoggiatura is a ornament that consists of an added non-chord note in a melody that is resolved to the regular note of the chord. For some reason, it is categorised as rhythm on Noteflight, so we'll just put it here. Also, it kind of looks like a grace note but without the slash on tail.
Duplet: Fitting two notes into one note.
Triplet: Fitting three notes into one note.
Quadruplet: Fitting four notes into one note.
Quintuplet: Fitting five notes into one note.
Sextuplet: Fitting six notes into one note.
Septuplet: Fitting seven notes into one note.

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Music theory + Composition Technique (FREE!)
Non-FictionThis book contains information on basic music theory for those wanting to introduce themselves to music and composition. It includes exercises and practices to help you learn quickly, all for free.