The current phonetic is NATO standard, but try to get the Canucks to say Zulu?  They always say Zed.

Regimental numbering system in the Army made lots of sense when it was adopted during WW1.  Before WW1 the Division was an ad hoc arrangement of regiments jumbled together as needed.  To prepare for the Great War the Army needed a more orderly system which was very simple.

As designed for WW1 the division was "square:"  Each Division had two Brigades.  Each Brigade had two Regiments (plus assorted troops).  This organization was for trench warfare to allow replacement of the two on-line regiments by the other two, or to pass thru two as needed.  Brigade and regimental numbers were based on the division number.

For example, the 101st Division of WW1 vintage (a National Army [draftee] division) was composed of the 201th and 202d Brigages composed respectively of the 401st & 402d, and 403d & 404th Regiments.  Pure math.

By this means you can usually determine the original division assignment of a unit.  Say the 145th Infantry Regiment.  This would have been in the 73d Brigade alongside the 146th Infantry Regiment (divide 145 by 2 and round up).  And part of the 37th Division (divide brigade number by 2 and round up).

These regimental numbers applied to the National Guard and National Army divisions and any new Regular Army regiments created for the war.  

The square division was abandoned circa 1940 and replaced by the triangular division (3 regiments, no brigades) that's basically in place today.  To do this the 4th regiment in each division was eliminated.  Since the National Guard was already federalized many of their regiments were simply reassigned outside the division.

Parachute, glider, tank destroyer, and other "non traditional" units were given regimental numbers outside of the WW1 system in many cases -- the 500-series of parachute units for example.  I think tank destroyers were in the 700 series.  They won't link to the original square designations.

Today many ARNG units still retain their historical association with a division or WW1 brigade.  Regular units are a hodge podge of regimental numbers with an attempt to keep those units on active duty with the better war records.

BTW, both the 82d and 101st divisions were "USAR" divisions between WW1 and WW2.

-- Chuck

Fire --

The Army changed unit designations promiscuously throughout history.  The oldest active regiment is the 3d Infantry.  Unlike Brit practice the numbers do not indicate precedence or "rank" among the regiments.  Oldest unit in the Army is a company (maybe battery) in MAARNG which has lineage to the early 1600s.  Only a handful of regular units have any participation credit for the Revolution; many east coast ARNG units have these streamers.

Regular regiments were numbered from the beginning, but units were disbanded or numbers just changed for the heck of it.  My old unit, the 35th Infantry can only trace it's lineage back to circa 1916 and at that time was one of the highest numbered regular regiments.

Before the creation of permanent divisions in WW1 all National Guard regiments had state numbers, such as 7th Ohio Infantry.  When these units were federalized during WW1 they were formed into divisions and given federal regimental numbers.  Few, if any ARNG regiments were at war strength when federalized, so several were often combined into a federal-numbered regiment.  I explained above how the regimental numbers were determined.

I've a list at home with the regimental-number blocks but here's my recollection:  Regular regiments were numbered 1 to 100; ARNG regiments started at 101; and I think National Army (later USAR) regiments started at 301.  Regular divisions ran to 25, ARNG to 74, and NA higher.  Note 82d and 101st were National Army divisions (draftees) in WW1 and reverted to reserve status after the war.

After WW1, when the ARNG was reformed the numbering system was maintained even for new units.  By WW2 it started getting jumbled again, and is only vaguely in place to day.

I mentioned streamers earlier.  Color bearing units (battalions or separate companies) are authorized campaign or battle streamers on their regimental colors.  The Civil War streamers can be either Blue over Grey, or Grey over Blue depending on USA or CSA service.  CSA service is considered to be US Army service and CSA earned battle honors accrue to current units.  These are all ARNG units in the states of the late Confederacy (to my knowledge).

-- Chuck
[Edited for typos]

Teddy Roosevelt reunited the Army when he ordered the return of Confederate regimental colors to the southern states.  Campaign steamers in Grey over Blue have been authorized at least as far as WW2, that's as far as I was able to research it, but I suspect they go back to WW1.  Wasn't there a movie called "Fighting 69th" where a New York ARNG regiment was brigaded with a Alabama ARNG unit?  

CSA battle honors use CSA battle names First Mannassas vs. First Bull Run for example.

Oldest US Army unit is in MAARNG and traces lineage to the original MA Bay Colony militia unit formed soon after the colony was founded.

Protocol demands that military units march in parades in order of longevity.  Eastern ARNG units can predate regular units by hundreds of years.

-- Chuck