There's no universal answer to this question. If we're lucky, the sheet music will tell us which direction we should strum or pick, but most sheet music doesn't.

There is a basic system that will help you decide between an upstroke or a down stroke. The system requires that you know how to read rhythms and count a measure. Here are some rules:

1. Use a downstroke on the beat.

2. If eighth notes are your fastest notes in the measure, use upstrokes for half-beats.

3. If sixteenth notes are your fastest notes in the measure, use upstrokes for quarter beats and three-quarter beats. Downstrokes on the beat and half beat.

For example, if you counted the rhythm as:
1 2 3 & 4
your alternate picking would look like this:
d d d u d

If you counted the rhythm as:
1e&a2 &a3 4
your alternate picking would look like:
dudud dud d

I hope this helps clear up some the of the confusion.

Earle Wood
www.ewguitar.com