> English is not my first language; is a constabulary not a police?

Totally understandable - English _is_ my first language, and the terminology is a clusterfuck to the point where nobody can clearly talk about it (which at times has seemed intentional, with terminology used in legislation on this topic, etc).

But I think when most people describe the different modalities of law enforcement, in terms of western common-law they mean:

* Watch is unranked, unarmed, discretionary, no special arrest power, volunteer, reporting to a public house or meeting of some kind

* Constabulary is unranked, sometimes armed, non-discretionary, no special arrest power, professional, reporting to the executive branch

* Police are ranked, discretionary, professional, armed, have special arrest power, reporting to the executive <=- in the USA, these began as, and were for a time called, "slave patrols"

* Sheriff are unranked, non-discretionary, professional, armed, have special arrest power (but relating only to court matters such as arresting a fugitive), directly elected but reporting to the judiciary

(when I saw "unarmed", I include what is sometimes called "privately armed", meaning that participants may carry privately-owned arms)

These terms have became all muddled, though. Particularly, police and sheriff have become nearly indistinguishable in terms of the function in US society.

> If you were relying on random individuals voluntarily (and without pay) doing the enforcement, a gang would only need to be sufficiently feared, so that noone ever dares touch them.

It seems to me that every place in the world where this configuration exists, there _are_ police. And the police are either paid off, or they are themselves the gang that is universally feared.

And again, it's possible to have a constabulary that is paid, but which does not have special powers (ie, can only arrest people using ordinary citizens arrests) and which is privately armed.

> Such units have existed for much longer than 200 years. But perhaps you use a different definition for the term?

For the vast majority of documented history of common-law, law enforcement in the general civilian public has been the purview of private groups and not the state.

Police, as they are today, meaning:

* Ranked and compelled to follow orders from superior officers
* Armed by the state
* Paid by the state
* Given lawful authority to make arrests in situations where other citizens aren't
* Given protections or exemptions from criminal and/or civil laws for activities during the course of their duties
* Staffed by civilians (eg, not subject to a code of military justice or to judgment by a court marshal)

...is not a long-standing institution in the western-common law tradition. Nor is the use of the word "politic" or "police" to describe such a force. The earliest police forces in the US today were slave patrols, which adopted different terminology after the 13th amendment, but whose mission and powers remained fairly consistent (be reminded that slavery did not end with the 13th amendment, as it was - and is - still a legal criminal sentence).

Many places in the world still don't have forces that meet all of these criteria, even though they may borrow the word "police".