Mezzo staccato
Dots under a slur: sustained but lightly separated.
What it means
Mezzo staccato means “half detached” and designates a manner of playing between binding and separation: the notes are sustained but lightly set off from one another. Every note keeps the greater part of its value and ends shortly before the next.
The designation is used largely synonymously with portato; for string players, louré corresponds to it. The terms overlap, and their delimitation is handled differently in the literature — in all cases the same middle articulation is meant.
Within the spectrum of articulation it lies between non legato, which calls for only a slight separation, and staccato, in which the note is distinctly shortened and an audible rest is left.
In the notation
The manner of playing is notated by dots or short strokes under a slur. Here the slur does not mean binding but belonging together; the dots indicate the separation. Strokes instead of dots call for the softer, more sustained execution. The distinction between the two is not handled consistently everywhere, and some editions use only one form.
Musical use
The manner of playing serves to present a melody as both articulated and connected at once. It is widespread in sustained movements, especially with repeated notes that would not be audible in pure legato, and in accompanying figures that are to carry without merging. It also occurs in vocal music, when individual syllables are to be set off but not detached sharply.
On the piano, the articulation succeeds with a light release of the hand. For string players it arises through short bow interruptions. The character is sustained and yet lightly organized, neither bound nor sharply detached.
Performance practice
Shorten the notes only slightly — the character remains sustained, the separation is a nuance. Produce it through a light easing, not through lifting. On the piano it helps to hold the note until shortly before the next and to leave the weight in the arm. And check whether dots or strokes stand there before fixing the sharpness of the separation.