There are several forces to be considered when you hang your pictures.
Usually when your hanging system fails it is at its weakest point.

The areas where failures are possible relate to the:
• hook
• hanger
• wire

The hook could be any multitude of fixture that is fixed to the wall...a nail....a picture hook...hollow wall hanger etc.

The hanger could include D-rings, screweyes, staples, keyhole plates, security hangers etc.

The wire could be stainless steel, chain, nylon, kevlar or other material.
 

The picture hooks

If only one hook is use then the maximum force it has to bear is a downward force exerted by the weight of the picture.

Conversely, if two hooks are used and they are both level horizontally and placed an equal distance in from the edge of the frame then they have to bear half of the weight each....easy so far....but read on...
 

The real forces at play

What is not apparent is the force that is created by the degree of tension in the wire.

The tighter the wire the greater the force applied to the hanger through the wire.

The breaking strain of the wire and the durability of the hanger and the way in which the wire is attached to the hanger must all be able to sustain the forces which are developed when the picture is hung on the wall.

Tight wire equates to more force being exerted on the hanging points and hooks.

Imagine carrying two full buckets of water by your sides....not too difficult right!

But if you extended your arms out to horizontal so you formed a cross then the buckets feel much heavier.

This isn't just a feeling there are forces at play.

The wire on a picture frame is the same...it is all to do with angles and trigonometrical formulas.

To control these forces you need to increase the angle from the anchor point to the hook....there are two ways of doing this: slack wire and using two hooks