The gutter is a strip on the left side of the text area that displays line numbers. The gutter is hidden by default; to make it visible, double-click on the thin vertical line at the left of the text area, or invoke View>Gutter Expanded (keyboard equivalent: Control-').
Lines containing markers or position registers (see the section called Markers in Chapter 4 and the section called Position Registers in Chapter 4) are drawn differently in the gutter.
The gutter can be resized by dragging with the mouse. The View>Save Gutter Size command can be used to make the gutter's current size the default for future sessions.
Many options relating to the gutter (colors, behavior, etc) can be changed in the Text Area and Colors & Styles panes of the Utilities>Global Options dialog box.
Figure 3-10. The gutter