Updated 25 Dec 2005
This page illustrates some common speaker wiring topologies and also shows various characteristics for the total system. Here are some important notes regarding the info on this page:
The figure below shows the wiring for two identical speakers in series:
This wiring is often used with two 4 ohm speakers (bringing the total system impedance to 8 ohms).
Series Wiring of Two Identical Speakers 

For 8 ohm speakers 
For 4 ohm speakers 

System Impedance 
16 Ohms 
8 Ohms 
System Power Capacity 
Twice any one speaker 
Twice any one speaker 
One thing to keep in mind with this wiring configuration: if one speaker blows (resulting in an open circuit), the second speaker will go silent (like the old fashioned Christmas lights, "if one goes out they all go out").
The figure below shows the wiring for two identical speakers in parallel:
This wiring is often used with two 8 ohm speakers (bringing the total system impedance to 4 ohms).
Parallel Wiring of Two Identical Speakers 

For 8 ohm speakers 
For 4 ohm speakers 

System Impedance 
4 Ohms 
2 Ohms 
System Power Capacity 
Twice any one speaker 
Twice any one speaker 
The figure below shows the wiring for four identical speakers in parallel:
This wiring may used with four 8 ohm speakers (bringing the total system impedance to 2 ohms).
Parallel Wiring of Four Identical Speakers 

For 8 ohm speakers 
For 4 ohm speakers 

System Impedance 
2 Ohms 
1 Ohm 
System Power Capacity 
4x any one speaker 
4x any one speaker 
The figure below shows the wiring for four identical speakers in seriesparallel configuration:
This wiring may used with any 4 identical speakers and the total system impedance is the same as that of a single speaker!
Series  Parallel Wiring of Four Identical Speakers 

For 8 ohm speakers 
For 4 ohm speakers 

System Impedance 
8 Ohms 
4 Ohms 
System Power Capacity 
4x any one speaker 
4x any one speaker 
There are many ways to wire up speakers, the ones shown above are the most common ones in use today. The main thing to keep in mind: when wiring up multiple speakers, be aware that undesireable problems can come up if the speakers are not the same type.