The normal snap is used to draw lines perpendicular to the edges of existing objects. The normal snap check box is displayed under 'Direction snaps' in the 'Drawing Options' data panel when the various drawing commands are active. When the normal snap is switched on and you draw a line string, each line will automatically snap to be perpendicular with it's predecessor if it exists:
To draw a block perimeter line to be perpendicular with the edge of an existing block, move the mouse cursor to touch the edge and then move it to be approximately perpendicular to it, the cursor will automatically snap to a line which is exactly perpendicular.
You can lock the perpendicular snap by pressing the SHIFT key which will prevent the cursor from picking up an unwanted direction inference. In this case, a construction line has been placed parallel with the block edge to help in positioning the new block. The perimeter line locks to a perpendicular from the construction line:
When the normal snap is in operation, intersections are automatically calculated with drawing guides, allowing you to snap to the intersection:
