Clans are families, but they are so much more than that.

There are a few things that should be kept in mind before you read this article.
First, I’m not an expert, this is simply me tracking my own thought process on the subject.
Second, this article is built on a few suppositions, which are::

• In a clan, the clan is prioritized over the individual. Sometimes, to such an extent where “individual” doesn’t really describe anyone.
• The clans are organized in some way. This implies they have “membership rules”, they keep track of “members” and have some “clan head” or “clan elders”.

Clans are one of the oldest, if not the oldest, form of hierarchy. Clans are an evolution of the familial unit (Note: I am not referring to “nuclear families”, which are a fairly recent invention. I am, broadly, referring to an “extended family”.), a more organized form of family.
Exactly how they form, I can’t be completely sure, but I can make a few assumptions.

1. The family starts to recognize a family head, who later chooses a successor to take over after him.
2. The family start keeping track of members.

If I come up with others, I will add them.
Clans may then further evolve into “tribes” which will simply be larger clans. Alternatively, a tribe can devolve into a clan again, if certain conditions are met.

These are families that have informally or unintentionally done something that could transition them into a clan.
Many families, especially extended families, will have an established “head”. However, this will almost certainly be an informal position. These are “Proto-Clans”. The head acts as a “core” of the family that acts as a unifier of several smaller nuclear families, usually because they are their grandparent/great-grandparent. Once they die, however, the proto-clan is unlikely to remain united. They share many features with pseudo-clans, so I see no point in describing proto-clans.
A Pseudo-Clan is formed when, for whatever reason, a proto-clan’s head dies, but the proto-clan remains united. These pseudo-clans can be considered a neutered form of the regular clan. They usually have a large network of people and great trust between members. They also meet semi-regularly. However, the individual and nuclear-families are more important than the clan as a whole. Individuals and nuclear-families can “leave” the clan at any time, without suffering many negative consequences. They may also “return” at any time. Pseudo-Clans only hold power if one or more members hold power, the clan itself is not powerful.

Clans all broadly operate in the same way. They have members dedicated to specific tasks. Those tasks usually include keeping track of members, punishing members, investigating members, arranging deals with other clans and “maintaining” the clan.

This is much easier than it may sound at first. All it requires is maintaining a list of all clan members, their place of residence (if they don’t live near the clan) and, sometimes, their position within the clan (or “rank”).
In a small clan, this duty can be performed by the head of the clan. For larger clans, it may be necessary to have a few extra members perform this duty as well.

Clans will tend to have an informal set of rules and guidelines governing them. Whether they write this down or maintain it through oral tradition is largely dependent on the clan and complexity of rules. For breaking these rules, members will be punished.
Severity of punishment, as well as who carries out the punishment, may depend on the transgression or the legal status of the rules andor clan. Usually, however, punishments would be carried out by the parents or siblings of the transgressor, or by the family head.
If a clan is large enough, they may have members dedicated to carrying out punishments.

This is a fairly minor task. This situation could only arise if the clan head, or high-ranking clan member, becomes suspicious of someone within the clan. You are unlikely to find any members dedicated to investigating other members as it’s likely to degrade cohesion.

This is a fairly simple task. These members are essentially “diplomats”. They will set up marriages, help merge clans if needed, negotiate with political entities and more.

This involves maintaining the traditions of the clan, performing duties as needed and making sure the “individual” never gains more importance than the clan as a whole.

In a clan, its members are usually defined as “members of the clan” rather than as individuals. You are judged, by the clan and outsiders (usually), as a member first and your own person second.
When talking with members of the clan, you are not talking to the individual member, but to the clan itself. When you insult a member, it is taken as an insult of the clan. When you praise a clan member, it is taken as praise of the clan.
To a clan, recognizing its members as individuals is a puzzling concept.
Within a clan, the situation is a little different. You are still judged as members of the clan first, but they do recognize you as an individual… within the clan of course.
Nuclear families are especially strange to a clan. To them, there is no difference between your children and the children of your brother. Functionally, most members are “cousins” or “siblings”.

When a clan grows large enough, it is sometimes hard to keep track of everyone. Especially when it is easy to split the members in some way.
For this reason, clans will usually divide into a group of several clans… with a clan head uniting them all.
The clan head will, usually, be chosen from the “main” branch of the clan. The other branches are then seen as side branches of the main branch.
Sometimes, it is possible for a side branch to replace the main branch and become the main branch. How this is done depends largely on the clan.
Side branches effectively function as smaller branches, with a few caveats. They would not be allowed to go against the main branch directly (unless of course, they end up replacing the main branch), they would have to listen to the main branch. Similarly, they wouldn’t have the “status” of the main clan, if such a status exists.

Clans are also a useful source of marriages. As the downsides of incestuous relationships effectively cease to exist once the distance is that of a cousin or greater… marrying members of the main branch with members of side branches is seen as advantageous. “Keeping it in the family” indeed.

Sometimes, members of clans will become disillusioned with the clan… or they desire to be seen as individuals rather than members of a clan… or a member commits a great transgression. Really, there can be many reasons.
In such an event, a clan member will want to leave or they’ll be kicked out.
The clan would, obviously, view this as a betrayal and would then punish the ex-member accordingly. The former member would no longer be allowed to call themselves members of the clan (this may extend to them loosing their surname), they would be treated poorly by the clan and, in extreme cases, they would not be allowed to meet with any lower-ranking members of the clan. In the most extreme cases (yes, more extreme than the previously mentioned one), the clan may break off engagements, marriages or expel any member who is “associated” with the ex-member.

On its own, the clan is essentially a mini proto-state. However, if it exists within a state… the situation changes. This of course depends on how much power they wield within a state, but in general, they can now specialise.
Does the state require high quality military officers? There is a clan renowned for their military geniuses. Does the state require skilled artists? One clan is world-famous for their artists, who produce incredible pieces of art. And so on…
Clans are especially useful in this regard as their members can effectively be taught from birth to be good at one thing. In a large clan, this advantage grows incredibly large as the most talented members would get ranked higher within the clan.
It is not a surprise that various states in various regions in various time periods relied on clans for this very reason.

Of course, this specialization cannot go on well forever. If a clan goes “rusty” they may only be a burden on the state that relies on them. When a clan goes decadent, then it will produce some of the worst members ever, not qualified for any position. In this case, the masses are a better source of recruits.

Members of clans, being family, would be loyal to each other. Loyal to a degree only soldiers and brothers can be. This is an incredible advantage for a clan, as the chance of betrayal is greatly reduced.
However, it is both an advantage and a disadvantage for the state housing the clan. Although clan members are unlikely to betray each-other, they do not share this loyalty for whatever state is in charge. If a clan is powerful enough, it is an existential threat to the status-quo.
In such an event, the state may attempt a purge… even if the clan proves, multiple times, their undying loyalty to the state. If the purge fails, the clan may attempt a coup.

Likely due to a combination of “Loyalties” and “Decadence”, clans are no-where to be seen in the political sphere. With greater centralization of the state and an improvement in education, the need for clans has shrunk significantly. However, the advantages of clans remain great. Even in the modern day, a competent and decently sized clan would likely dominated whatever field it decided to focus on.
The chance of their return, however, is slim.

This has been “Clans” and I honestly am not sure what to think about it.
I think I took the subject a bit too seriously… so I’m not sure about the writing style. I think I’ll be making small changes here and there, if only to make some sentences less awkward.
How is this useful to you, the reader though? Well… maybe you’ll understand why some families/individuals/groups act the way they do. Or, you may find it useful for worldbuilding. It’s up to you to figure out if this is useful or not.
Ha, maybe someone will see this and decide to start a clan? Yeah right~