The Appeal to Common Practice is a fallacy with the following structure:
X is a common action.
Therefore X is correct/moral/justified/reasonable, etc.
The basic idea behind the fallacy is that the fact that most people do X is used as "evidence" to support the action or practice. It is a fallacy because the mere fact that most people do something does not make it correct, moral, justified, or reasonable.
An appeal to fair play, which might seem to be an appeal to common practice, need not be a fallacy. For example, a woman working in an office might say "the men who do the same job as me get paid more than I do, so it would be right for me to get paid the same as them." This would not be a fallacy as long as there was no relevant difference between her and the men (in terms of ability, experience, hours worked, etc.). More formally:
It is common practice to treat people of type Y in manner X and to treat
people of type Z in a different manner.
There is no relevant difference between people of type Y and type Z.
Therefore people of type Z should be treated in manner X, too.
This argument rests heavily on the principle of relevant difference. On this principle two people, A and B, can only be treated differently if and only if there is a relevant difference between them. For example, it would be fine for me to give a better grade to A than B if A did better work than B. However, it would be wrong of me to give A a better grade than B simply because A has red hair and B has blonde hair.
There might be some cases in which the fact that most people accept X as moral entails that X is moral. For example, one view of morality is that morality is relative to the practices of a culture, time, person, etc. If what is moral is determined by what is commonly practiced, then this argument:
Most people do X.
Therefore X is morally correct.
would not be a fallacy. This would however entail some odd results. For example, imagine that there are only 100 people on earth. 60 of them do not steal or cheat and 40 do. At this time, stealing and cheating would be wrong. The next day, a natural disaster kills 30 of the 60 people who do not cheat or steal. Now it is morally correct to cheat and steal. Thus, it would be possible to change the moral order of the world to one's view simply by eliminating those who disagree.
This fallacy is closely related to the argumentum ad populum. It consists of asserting that the more people who support or believe a proposition, the more likely it is that that proposition is correct. For example:
"The vast majority of people in this country believe that capital punishment has a noticeable deterrent effect. To suggest that it doesn't in the face of so much evidence is ridiculous."
"All I'm saying is that thousands of people believe in pyramid power, so
there must be something to it."
Director Jones is in charge of running a state waste management program. When it is found that the program is rife with corruption, Jones says "This program has its problems, but nothing goes on in this program that doesn't go on in all state programs.""Yeah, I know some people say that cheating on tests is wrong. But we all know that everyone does it, so it's okay." "Sure, some people buy into that equality crap. However, we know that everyone pays women less then men. It's okay, too. Since everyone does it, it can't really be wrong." "There is nothing wrong with requiring multicultural classes, even at the expense of core subjects. After all, all of the universities and colleges are pushing multiculturalism."