The Measure Tool is used to gain knowledge about pixel distances in your working image. By clicking and holding the mouse button, you can determine the angle and number of pixels between the point of click and where the mouse pointer is located. The information is displayed on the status bar or can also be displayed in the Info Window.
When you pass the mouse pointer over the end point it turns to a move pointer. Then if you click you can resume the measure.
The Measure Tool can be called in the following order, from the image-menu:
/ .
The Tool can also be called by clicking the tool icon:
Ctrl key pressed while dragging the pointer performs a strict horizontal measure.
Alt key pressed while dragging the pointer performs a strict vertical measure.
Ctrl key pressed and click on the end point places a horizontal blue dotted guide.
Alt pressed and click on the end point places a vertical blue dotted guide.
Ctr-Alt key combination allows to move the measure.