1. Above, below, left, right and nothing else.
Alignment
Above, below, left, right and nothing else.

2. Alignment reduces every spatial relationship to above, below, left or right.
The real world is a three dimensional space. The difficulty with drawing is that it is by nature, a two dimensional space. It is impossible to make a three dimensional drawing, you can only make it look three dimensional. One of the key abilities you must become proficient at is to stop interpreting what you are drawing as three dimensional (even when it is.) One way to do this is to reduce all spatial relationships to above, below, left, right, and nothing else. This keeps all points in a two dimensional relationship, and therefore makes it possible to draw. There are three ways to use alignment as you draw.
above
below
left
right

3. First look for perfectly vertical or horizontal relationships between reference points
The first and most straightforward way to use alignment is to look for perfectly vertical and horizontal relationships between reference points. In this drawing there is a perfectly vertical relationship between the left side of the pointer finger and the right side of the ring finger. Also there is a perfectly vertical relationship between the top of the ring finger and the crease at the bottom of the pointer finger. If these points are not perfectly aligned in your drawing then you would know your drawing is inaccurate and needs to be adjusted.

4. Second look for near vertical or horizontal alignments
The second application of alignment is to look for point almost in alignment many important reference points will not line up perfectly with another reference point but you still apply the principle of alignment effectively. In this drawing of a hand the top of the index finger and the top of the thumb are in near horizontal alignment. If you were drawing this hand you could start with placing the thumb and then draw the top of the index finger slightly above it.

5. Third locate points in terms of above or below, and left or right
The third application of alignment is to think more generally about relative position. It can be helpful to start with the highest point and work your way down a series of reference points. In this drawing of a hand the index finger is above the pointer finger which is above the ring finger (just slightly,) which is above the pinky (by a significant margin,) which is above the thumb. It is amazing how helpful it can be to make this simple analysis. Of course this same principle can be applied horizontally as well.

6. Alignment is useful for setting up boundaries and correcting misplaced reference points
Alignment can be used when blocking in a rough shape to set boundaries for the shape. Mark the positions of the reference points at the top, bottom, left and right of what you are drawing. Then as you draw remember to draw inside those marks because those marks represent the furthest points of the drawing and nothing can be outside of the those marks.

7. Alignment works because it simplifies every relationship to two choices
The beauty of alignment is that it reduces every spatial relationship to one of two choices. Above or below, and left or right. In this drawing the index finger is above the pinky, and to the right. Alignment is an excellent tool for checking the relative position of near and far reference points. Every drawing done by a human will need adjustment as the drawing progresses. When you find a point out of alignment, fix it. Remember that every part of the drawing must be subject to change until every point is in the correct relationship to every other point.