When drawing new area or line features or moving
existing features, you can use the CTRL and SHIFT keys to cause the next
vertex to be snapped vertically or horizontally relative to the last vertex
or moved vertically or horizontally compared to the features current location.
Hold down the CTRL key to snap vertically, the SHIFT key to snap horizontally,
or both the CTRL and SHIFT keys to snap diagonally.
Selecting the option New Segments Are Horizontal or Vertical Lines in the Optionssubmenu will create new feature segments that are either horizontal or vertical to the previous.
The option Default to Right Angles to Previous Segment in the Optionssubmenu will set the default to using right angles from to the previous segment when creating new features with the Digitizer tool