Someone will thank me for this, eventually.
You can use two wire for the travelers also. Old electricians trick
To use only two wire for traveler, you start at light fixture box. Run "hot" to first switch, connecting hot to switch and making up grounds. Wire nut and tape off neutral (white) wire. Using both white (with label) and black along with ground connect to other switch . From second switch, run hot back to controlled fixture. Two wire is a LOT less expensive than three wire. This is especially important when you have multiple two way switches in a circuit.For this conversation and using existing (12-2 or 14-2 with ground) single switch wiring. You'll need a new length of 12-3 or 14-3 with ground. Run it to the new or existing box, wire as shown and install 2x 3way switches and a single wire nut and your in business.
Don't forget to tape the white wire with length of black electrical tape.so the next guy knows it's hot! I perfer to mark the white wire with a black sharpie myself.
To use only two wire for traveler, you start at light fixture box. Run "hot" to first switch, connecting hot to switch and making up grounds. Wire nut and tape off neutral (white) wire. Using both white (with label) and black along with ground connect to other switch . From second switch, run hot back to controlled fixture. Two wire is a LOT less expensive than three wire. This is especially important when you have multiple two way switches in a circuit.
Jeff, thanks for showing your diagram, which illustrates one of the more common situations where 3-way switches are added. When I wired my shop 15 years ago, I verified the wiring I needed by seeking good diagrams like the one you showed. There are many scenarios that are possible depending on where the source of power enters and if the user wants to have other elements such as receptacles or dimmer switches included. Here is a link to several scenarios where these options are diagrammed.
Neither of the 12-2 or 14-2 are code, running and extra wire would require the lone wire to be taped to the romex that powers the 3way. In either case most cites and counties would not pass it, it will work and actually really not a big deal, except to the inspectors.
The House I bought the previous owner was into both electronics and electrical, so this house has 4 - 4way and 4- 3way switches wired in it... God only knows why
@Rkadish I used Rob's (@McRabbet) link 3 Way Switch Wiring Diagrams - Do-it-yourself-help.com to find this:do you have a diagram for a 4 way?