Series and Parallel Pickup Wiring
Most guitars with more than one pickup installed will let you choose to play through any one pickup by itself or through combinations of all pickups. The standard way for muli-pickup guitars to connect their multiple pickups together is known as Parallel Wiring. There are many sites on the www where you can learn about Parallel wiring and how/why it works as it does; do a search for "parallel series guitar" and you'll find all the information you want.
The way I understand it is that the pickup signals are combined alongside each other on their way from your controls to the output jack. This combining is done at your pickup selector switch. Reading left to right::
You can have the 20 if you want it by connecting the pickups in Series rather than in Parallel. Series wiring connects the pickups end to end so to speak:
Here are some small MP3s that demonstrate the difference between Parallel and Series wiring. You will first hear the neck and bridge pickups in parallel, then the neck and bridge pickups in series.
Here are some examples of how to add the option of Series coupling to your guitar. Use a Push-Pull volume/tone control to add a 'mini-switch' to the guitars without having to drill any holes. When the pot/switch is down, you'll have standard Parallel wiring. When the pot/switch is pulled up the pickups are connected in Series. An added benefit with Series coupling is that the Tone controls will act independently of each other. In Parallel, turning one Tone control down kills the entire tone. In Series, turning one Tone control down affects that pickup only.
Parallel/Series Wiring for Les Paul type guitars
This mod will allow Series coupling of the Neck and Bridge pickups
Parallel/Series Wiring for Statocaster type guitars
This mod will allow Series coupling of the Neck and Middle pickups and of the Middle and Bridge pickups.