When using the polygon or freehand tools, you can also use the shift key to add to selections. Line Selection Tools - Use these tools to create straight, freehand or segmented line selections. With straight lines only, you can dynamically vary the line width by clicking on the lines at the bottom of the tool palette. Freehand line selections are created in the same way as freehand region selections, except they are not required to be closed. Segmented line selections are created using a technique similar to the way polygon region selections are made, using a double-click to terminate the line.
Line width is specified by clicking on the lines at the bottom of the tool palette.
Straight lines can be constrained to be either vertical or horizontal by holding down the shift key. Line lengths recorded using the Measure command are given in pixels unless Set Scale has been used to perform spatial calibration. Use Show Results to display the list of length measurements. Option-click with the text tool to label lines with the measured length. Double-click on the line selection tool to bring up the Set Scale dialog box.