A custom is a traditional way of behaving that is specific to a particular society, and they will vary according to the society you are talking about, and over time. Customs may also vary within a society: different social groups and different regions may have different customs.
Focussing on the United Kingdom, while it is easy to provide a list of 'national customs' such as eating a roast dinner on Sunday, drinking tea, obsessing about the weather, supporting the queen, 'white weddings', loving the NHS etc. , these might just be stereotypical ideas about 'Britishness' rather than actual norms and values that people subscribe to.
This is because we live in a complex, diverse, multicultural society, in which customary ways of behaving are increasingly questioned by more and more people.
A related concept to 'social customs' is 'social norms'... they are basically the same thing, except that the word 'custom' is the more commonly used word, and customs tend to be linked to 'traditions' and are thus not reflected on or thought about very much in day to day life.
If you're interested in finding out more about social norms... please this post....
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