Alternate Picking

The Technique

Alternate picking is a way of picking where the pick hits the strings with upstrokes and downstrokes mixed. The mixing is always in the same order, so it's down-up-down-up. It's never down-down-up-up. When switching strings the movement just goes on. If people want to go double down or double up, when changing strings, they use economy picking. Here's an example of alternate picking:

E------------------------------------------------------------------
B------------------------------------------------------------------
G---6 (down)--7(up)--6(down)--7(up)--------------------------------
D------------------------------------5(down)--8(up)--7(down)-------
A------------------------------------------------------------------
E------------------------------------------------------------------

Use

Many guitarists use this technique to play through small intervals at speeds which would be impossible to reach with downpicking. It is considered on of the main shred techniques. It is not so commonly used for riffing though as downpicking is believed to give a heavier sound. Power Metal especially though, uses alternate picking a lot in riffing, to repeat a single note very fast. This is also known as tremolo picking.

Variations

String Skipping

String skipping is alternate picking, except you jump over strings, i.e. you play three notes on the A string, then three on the G string without playing anything on the D string. Paul Gilbert uses this technique a lot to alternate pick arpeggios.

Economy Picking

Economy Picking is a cross between alternate picking and sweep picking. It is thought to be faster and have a smoother sound, though there is debate as to whether it is better than alternate picking or not. You can see an article about it here.

Related lessons

Kristofer Dahl has done a Alternate/Speed picking series, you can watch the basic lesson Here