In musical notation, staccatissimo (plural: staccatissimi or the anglicised form staccatissimos) indicates that the notes are to be played extremely separated and distinct, a superlative staccato. This can be notated with little pikes over or under the notes, depending on stem direction, as in this example from Bruckner's Symphony No. 0 in D minor:
Alternatively it can be notated by writing the word "staccatissimo" or the abbreviation "staccatiss." over the staff. A few composers, such as Mozart, have used staccato dots accompanied by a written instruction staccatissimo when they mean the passage to be played staccatissimo.
See also 
- Philip Farkas: The art of French horn playing. Alfred Music Publishing 1956, ISBN 0-87487-021-6, p. 51 (online copy at Google Books)