Laws that govern usability

Paraphrased and extracted form Wikipedia:

Fitts' law: The time required to rapidly move to a target area is a function of the distance to and the size of the target. Fitts's law is used to model the act of pointing.

Consequence, use big areas and try to keep them close to preceding action area. If it is not possible try to increase distance between conflicting hotspots, to help preferred areas.

             BAD                                    BAD
Content 1: ----------  o              Content 2: ----------         [  ]
           ----------                            ----------


             GOOD
Content 3: ----------  [  ]
           ----------

Accot-Zhai_steering_law: It is easier/faster to navigate through a wider tunnel than a narrower one. Also, it is easier to navigate through a tunnel with thinner wall(s).

Consequence, for fluid motion - enable more space. That is don't force the user to perform orthogonal tasks (straight drags) provide margin of action.

Example 1:

----------     Hard          ----------
|        |    -- \           |        |
|        |    -- /           |        |
----------                   ----------

Dragging/moving from left to right, is harder in above case than that in below.

-----               -----
|   |    Easy       |   |
|   |    ---\       |   |
|   |    ---/       |   |            
|   |               |   |
|   |               |   |
-----               -----

Laws of crossing: It is easier to cross an object in a perpendicular direction. Crossing is an action which looks like a strike (drag) and is considered to be easier to achieve than pointing.

Using past illustration, crossing (top to down) is harder in Example 2 than in Example 1.