Bow stroke
Type and direction of the bowing.
What it means
The bow stroke covers two things: the direction in which the bow is drawn and the manner in which it treats the string. Direction includes down-bow and up-bow; manner includes bowing styles such as détaché, legato, martelé, staccato, or spiccato. Together they determine the attack, length, volume, and colour of a note — on a string instrument, what tongue and breath accomplish on a wind instrument.
In the notation
The bow stroke is indicated by the direction signs above the notes, by slurs that group notes belonging together into one stroke, and by dots, strokes, and wedges for articulation. On strings, a slur always also means: everything in one bow direction. Additional words such as sul tasto or am Steg concern the place of bowing and likewise belong to the editing of the part.
Musical use
Because a bow is of finite length, the distribution of the bow has direct musical consequences: it determines where phrases breathe and where accents fall. Composers and editors therefore prescribe bowings, and orchestral parts are set up uniformly by the section leader. The conception changes historically: Baroque music works with many short, articulated strokes and a distinct heavy–light contrast; Romanticism tends to aim for long, uninterrupted bow lines. There are no fixed prescriptions, but there is the rule that the stroke must serve the phrase, not the other way round.
Performance practice
Plan a passage backwards: decide which note needs the greatest emphasis, and divide the strokes so that it falls at a comfortable point. Write the chosen directions into the part, otherwise you will play differently next time. Practise a passage at three tempi and observe whether the distribution still works when fast — what is comfortable slowly can use up the bow at tempo. Watch for a steady contact point between bridge and fingerboard; most unclean notes arise there and not in the left hand. In the orchestra, the bowing marked in the part applies, even if you would privately choose differently.
Also check whether the bowings marked come from the composer or from the editor, because this determines how freely you may change them.