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To view the crossfader click the grey X icon (turns yellow) displayed in the lower right side of the session view.

Right click (Control-click) to set the style of cross fade.

A and B buttons

The A and B buttons at the bottom of each channel represent the side of the crossfader that will be sent to the Master out (heard by the audience) . A is for left and B is for right. If setting up a Cue Mix in Ableton Live the A and B buttons determine which side of the crossfader represent the "Master out" with the other being the Cue