I recently noticed I have heard guitar players talk a fair bit about coil taps but don't actually know what it means for a guitar to have coil taps?
What is the science behind it and why would a person want coil taps in a guitar?
There's a very good description of coil tapping in this article on the Seymour Duncan site:
Coil tapping is when a wire runs off of the pickup windings at a certain point, somewhere short of the full amount. This means you can install a switch to select between a single coil pickup’s full output or a lower output, giving you two distinct levels of power from one pickup.
This differs from coil splitting, described as:
Coil splitting is the practice of shutting off (or otherwise fading out) one coil of a humbucker, leaving behind a single coil for a brighter tone. Coil splitting is often confused with a single coil option known as coil tapping
Although I've often heard people referring to coil splitting on humbuckers as 'coil tapping'.
The reason why you would want taps is to emulate single coils or to get phasing effects. For example, humbuckers are wound so that one coil is the reverse of the other. On a stratocaster, the middle pickup is also reverse of the neck and bridge. On a strat, the 2 and 4 switch positions mix the neck or bridge pickup with the out-of-phase middle pickup and create a 'quack'-like sound. To get this with humbuckers, you could have a bridge and neck humbucker and a single coil middle and use coil taps or coil splits to split a single coil from the neck or bridge to mix with the middle. There are lots of other variations.