The Ruler tool helps
you position images or elements precisely. The Ruler tool calculates
the distance between any two points in the workspace. When you measure
from one point to another, a nonprinting line is drawn, and the
options bar and Info palette show the following information:
The starting location (X and Y)
The horizontal (W) and vertical (H) distances traveled from the x and y axes
The angle measured relative to the axis (A)
The total length traveled (D1)
The two lengths traveled (D1 and D2), when you use a protractor
All measurements except the angle are calculated in the unit of measure currently set in the Units & Rulers preference dialog box.
If your document has an existing measuring line, selecting the Ruler tool causes it to be displayed.