six different ranks of Sergeant (excluding the Three Seargent Major ranks)
Here lies the answer to Shek's question!
Basically American and other Prussian-model mid-level NCOs (i.e. the ones instrumental in running the infantry coy) are lot more
secure in their positions and share a greater deal of responsibility with their officers. Military law and tradition gives them lot more protection (from officers) but expects them to bear lot more responsibility in return. Strong NCOs means that you can let relatively younger officers try their hand at large commands.
Conversely mid-level NCO appointments in British-style armies were at the discretion of the officers. So first of all the NCOs were reluctant to bear too much responsibility because of liabilities (yes sorr, no sorr, three bags full sorr!), so you needed a stronger and more experienced officer to take up the bigger portion of responsibilities. Then not only was maturity needed to handle the ranks, but also extra maturity was now needed to handle NCOs because the officer could easily make or break NCOs (since many NCO appointments are discretionary). That atleast was how it evolved unti 1940s. As I understand the mid-level NCOs in British Armies became much more assertive in late 1940s (to the point of almost forming a union!), but the legacy of Majors commanding inf coys lives on.
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Don't kow how much impact colonial operations had on the this legacy at all: For example the biggest of all colonial armies, the British Indian Army, operated on a completely different system of command. All officers were, by default, Staff Officers. A completely different rung of Indian commanders called "Viceroy's Commissioned Officers" held between NCOs and COs, but unlike WOs held full command (usually upto company level, but sometimes batt. level too). So on and so forth. The British Army tried its best not to pick up anything from the colonial armies unless forced to by developments in Europe (ex. camouflage, staff schools, fixed logistics - all pioneered by the BIA and latd frced upon the BA).
PS: Do WOs hold command in some units?