George Orwell

The Seven Commandments - written on the barn wall - are the basic principles of animalism and described originally as "unalterable laws" by which the animals were to live. They were meant to keep the animals equal and to ensure that all animals were true to their own nature.  Over time, the commandments begin to change; they have addendums unexplainably added to them.  The animals see what the commandments say and question their own memories.

Almost immediately after the Commandments are written the cows’ milk and the windfall apples are taken by the pigs exclusively; so the seventh Commandment (“All animals are equal”) seems to be undermined from the very beginning.

The first two Commandments (“Whatever goes on two legs is an enemy” and “Whatever goes on four legs, or has wings, is a friend”) are subtly broken in the first years of Animal Farm but there is no attempt to rewrite them. Snowball, the hero of the Battle of the Cowshed, becomes an enemy of the farm after his expulsion by Napoleon, while there is resumption of trade via Mr Whymper.

The first alteration to the Commandments comes after the pigs move back into the farmhouse. The ban on sleeping in beds in the fourth Commandment is changed in Napoleon's favour by the addition of the words "with sheets". The next alteration to the Commandments is far more shocking. After the failures of the winter and the collapse of the first windmill, the pigs use Snowball as scapegoat. This leads to the 'show trials' in which animals 'confess' to crimes inspired by Snowball. The horrific executions that follow are in direct contradiction of the original sixth Commandment (“No animal shall kill any other animal”) but when this is checked the words "without cause" have been added.

Napoleon's hangover is a cause for alarm but all that eventually happens is that the words "to excess" are added to the fifth Commandment (“No animal shall drink alcohol”).

No alteration is ever made to the third Commandment about wearing clothes because by that time the pigs are so powerful that it is unnecessary. Instead, all pretence of "unalterable laws" is abandoned and the Commandments are replaced by the meaningless slogan:

"All animals are equal, but some are more equal than others." 

The Seven Commandments are a successful way of tracing the decline of the rebellion because they show how the pigs alter the rules on the farm to suit themselves. The alterations to the Commandments are sometimes funny, like the one about alcohol, and sometimes shocking, like the one about killing.