The pigs Snowball, Napoleon, and Squealer adapt Old Major's ideas into 'a complete system of thought', which they formally name Animalism. Soon after, Napoleon and Squealer indulge in the vices of humans (drinking alcohol, sleeping in beds, trading). Squealer is employed to alter the Seven Commandments to account for this humanisation, an allusion to the Soviet government's revising of history in order to exercise control of the people's beliefs about themselves and their society.
The original commandments are:

Whatever goes upon two legs is an enemy.
Whatever goes upon four legs, or has wings, is a friend.
No animal shall wear clothes.
No animal shall sleep in a bed.
No animal shall drink alcohol.
No animal shall kill any other animal.
All animals are equal.


Later, Napoleon and his pigs secretly revise some commandments to clear them of accusations of law-breaking (such as "No animal shall drink alcohol" having "to excess" appended to it and "No animal shall sleep in a bed" with "with sheets" added to it). The changed commandments are as follows, with the changes bolded:

4 No animal shall sleep in a bed with sheets.
5 No animal shall drink alcohol to excess.
6 No animal shall kill any other animal without cause.

Eventually these are replaced with the maxims, "All animals are equal, but some animals are more equal than others", and "Four legs good, two legs better!" as the pigs become more human. This is an ironic twist to the original purpose of the Seven Commandments, which were supposed to keep order within Animal Farm by uniting the animals together against the humans, and prevent animals from following the humans' evil habits. Through the revision of the commandments, Orwell demonstrates how simply political dogma can be turned into malleable propaganda.