The Post window is SuperCollider's primary feedback channel for results, warnings, and errors

In the SuperCollider IDE (whose three main panels are the Code Editor, Help Browser, and Post window) the Post window displays the response to every evaluated expression: computation results, notifications, warnings, error messages, and server boot status. It is the main visual feedback channel, and SC reports errors there before making any sound, so keeping it visible is essential for debugging. SC automatically posts the result of the last evaluated expression even without an explicit .postln call. A common beginner surprise follows from this: evaluating several lines auto-posts only the last one’s value (plus any explicit .postln output), which is also why a "Hello World".postln; line appears to print twice — once from postln, once from the last-thing rule. Clear the Post window with ctrl+shift+P.

Examples

"Hello World".postln; // appears twice: once via postln, once as the last result
2 + 2; 3 + 3;         // only 6 is auto-posted (last evaluated expression)

Evaluating a Pbind posts the EventStreamPlayer object reference.

Assessment

Predict what the Post window shows when you evaluate "A".postln; "B".postln; 2 + 2;, then verify and explain the rule. Explain why "Hello World".postln; appears twice, and what 3 + 3; shows with no postln call.