Whether a stem points up or down does not affect the note length at all. There are two basic ideas that lead to the rules for stem direction. One is that the music should be as easy as possible to read and understand. The other is that the notes should tend to be "in the staff" as much as reasonably possible.
Basic Stem Direction Rules
- Single Notes - Notes below the middle line of the staff should be stem up. Notes on or above the middle line should be stem down.
- Notes sharing a stem (block chords) - Generally, the stem direction will be the direction for the note that is furthest away from the middle line of the staff
- Notes sharing a beam - Again, generally you will want to use the stem direction of the note farthest from the center of the staff, to keep the beam near the staff.
- Different rhythms being played at the same time by the same player - Clarity requires that you write one rhythm with stems up and the other stems down.
- Two parts for different performers written on the same staff - If the parts have the same rhythm, they may be written as block chords. If they do not, the stems for one part (the "high" part or "first" part) will point up and the stems for the other part will point down.This rule is especially important when the two parts cross; otherwise there is no way for the performers to know that the "low" part should be reading the high note at that spot.