Hit [ctrl+M] to open up the Meter window. It shows the levels of all inputs and outputs (in figure \ref{fig:scaudiobus}, you see eight inputs on the left, and eight outputs on the right.

Now try the following line of code:

{Out.ar(1, SinOsc.ar(440, 0, 0.1))}.play; // right channel

The Out UGen takes care of routing signals to specific buses.

The first argument to Out is the target bus, that is, where do you want this signal to go. In SuperCollider, audio buses are numbered from 0 to 127 (see figure \ref{fig:scaudiobus}. The first eight (0-7) are by default reserved to be the output channels of your sound card. The next eight (8-15) are reserved for the inputs of your sound card. All the others (16 to 127) are free to be used in any way you want, for example, when you need to route audio signals from one UGen to another. In the example above, the number 1 means that we want to send the signal to bus 1, which is the right channel of your sound card.

The second argument of Out.ar is the actual signal that you want to “write” into that bus. It can be a single UGen, or a combination of UGens. In the example, it is just a sine wave. You should hear it only on your right speaker (or on your right ear if using headphones).

With the Meter window open and visible, go ahead and change the first argument of Out.ar: try any number between 0 and 7, and watch the meters. You will see that the signal goes wherever you tell it to.

TIP: Most likely you have a sound card that can only play two channels (left and right), so you will only hear the sine tone when you send it to bus 0 or bus 1. When you send it to other buses (3 to 7), you will still see the corresponding meter showing the signal: SC is in fact sending the sound to that bus, but unless you have an 8-channel sound card you will not be able to hear the output of buses 3-7.