

Click and drag to draw a line.
The
mouse button draws in the foreground color. The mouse button draws in the background color.You can also set the line width.
Hold Ctrl to draw lines angled at the nearest multiple of 30 degrees - these are the lines in the red diagram.
Hold Shift to draw lines angled at the nearest multiple of 45 degrees - these are the lines in the blue diagram.
Hold Ctrl and Shift to draw lines angled at the nearest multiple of 30 or 45 degrees - these are the lines in the green diagram.