Why so many different string trees and...

Why are they so different? I can get changing from the original to the modern, but mine has 1 that is screwed down to the headstock, some have 1 with a spacer, some have 2...So should I add a spacer or a second one to my MIM Standard? Or does it really matter?
Rob

It most often depends on the tuner and the height the last wind of the string (given an 'ideal' number of turns) leaves it at which determines the height/type you need for a string tree. It is all about optimizing the break angle over the nut, and to make the strings run at roughly the same angle for the larger part of their path to the tuning post. For instance, it is 'possible' to do without a string tree for the high E and B by winding more string onto the tuner and lowering the string angle relative to the other strings. But then you get tuning instability by having to many windings, so a string tree is the best overall solution.

One of the main functions of the string tree is to create the proper break angle as the string passes the nut on to the tuner. So, the placement of the string tree and it's height are somewhat subjective. You can put it just about anywhere up and down the face of the headstock and have it as tall or as low as you need. However, what you're trying to achieve is an angle similar to the angle you get on the lowest strings (E and A). Then again, you might want to keep the angle shallow to prevent binding if you do a lot of trem arm bends or behind the nut bends. It really just depends on your situation. I usually take a guitar that's already set up the way I like it and use that as a model for my projects.

Dan Earlewine's books on setting up guitars (from StewMac) has a section that talks about this.

I do think the break angle seems more extreme on my 1st and 2nd than on my 6th, but my Standard came with no spacer and seems to play and stay in tune just fine so I guess I won't mess with it!
Rob