In the interest of historical accuracy, some corrections to the following are inserted.
not to open a can of worms, but there were earlier periods that complicates things.
In 1923, the Department of Commerce created a new top-level license, the Amateur Extra First Grade, that conveyed extra operating privileges. It required a more difficult written examination and a code test at twenty words per minute.
The Amateur Extra First Grade of 1923 also required a year's experience and a clean record (no violations).
Only a few of them were earned, because the extra operating privileges weren't anything most hams wanted back then.
In 1933, the Federal Radio Commission (FRC) reorganized amateur operator licenses into Classes A, B and C.
Class A conveyed all amateur operating privileges, including certain reserved radiotelephone bands. Amateur Extra First Grade licensees and Amateur First Class licensees with "unlimited radiotelephone" endorsements were grandfathered into this class.
Class B licensees did not have the right to operate on the reserved radiotelephone bands. Amateur First Class licensees were grandfathered into this class.
Class C licensees had the same privileges as Class B licensees, but took their examinations from other licensees rather than from Commission field offices. Because examination requirements were somewhat stiffened, Temporary Amateur licensees were not grandfathered into this class but had to be licensed anew
In those days, the US amateur HF/MF bands were 160, 80/75, 40, 20 and 10. 40 was all-CW, no phone allowed. The "reserved radiotelephone bands" were 75 and 20 'phone, which required Class A. Class B and C could only use 'phone on 160, 10, and VHF/UHF.
Then in 1951, the FCC moved to restructure the existing three license classes (A, B, and C) into six new classes: Novice, Technician, General, Conditional, Advanced, and Amateur Extra. Each license class required two exams, one on theory and one on Morse code, and each license was valid for five years (except Novice). Until the advent of incentive licensing in the late 1960s, the Technician, Conditional and General classes shared the same written examination and the Conditional, General, Advanced and Amateur Extra classes shared the same operating privileges.
No, that's not right.
Here's how it worked in 1951:
Novice was a new 1-year one-shot introductory license with very limited privileges. 5 wpm code and a simple written test.
Technician was a new 5 year license meant for experimenters. Full privileges on 220 and up, no privileges below. 5 wpm code and the same written test as Conditional and General
General was the old Class B, renamed. 5 year license, full privileges except no phone privileges on the bands between 2.5 and 25 MHz. 13 wpm code and the same written test as Conditional and Technician. FCC exam only.
Conditional was the old Class C, renamed. 5 year license, full privileges except no phone privileges on the bands between 2.5 and 25 MHz. 13 wpm code and the same written test as General and Technician. Exam by mail.
Advanced was the old Class A, renamed. 5 year license, full privileges. Required holding a General or Conditional for at least 1 year, plus an added written test. If the prospective Advanced had a Conditional, s/he had to pass 13 wpm code and the same written test as Conditional and Technician at an FCC exam session before being allowed to try for Advanced. FCC exam only.
Amateur Extra was a new 5 year license, full privileges. Required holding an Advanced, General or Conditional for at least 2 years, plus 20 wpm code and an added written test. If the prospective Extra had a Conditional, s/he had to pass 13 wpm code and the same written test as Conditional and Technician at an FCC exam session before being allowed to try for Extra. FCC exam only.
The new Amateur Extra was intended to replace the Advanced as the top license. No new Advanceds would be issued after December 31, 1952.
The Advanced class was earned after the General Class through passing the Element 4A theory exam. Class A operators were assigned Advanced Class licenses following the 1951 structure decision.
Pretty much correct, except that the Advanced required 1 year experience, and I don't think the theory exam was called 4A back then.
Although existing Advanced Class licenses continued to be renewed, new licenses were not issued in the period 1951–1967.
Wrong date. The Advanced was closed to new issues at the end of 1952, and was reopened in 1967.
As things stood after the 1951 restructuring, anyone who wanted HF 'phone on the bands between 2.5 and 25 MHz would have to get an Extra if they didn't get an Advanced before the end of 1952. This caused a number of hams to get Advanceds before they became unavailable.
But then, near the end of 1952, FCC did a complete 180 and gave full privileges to Generals and Conditionals, effective mid-Feb 1953. For the next 15 years, there were 6 license classes in the USA (Novice, Technician, General, Conditional, Advanced and Amateur Extra) and four of those classes had full privileges. Only Novices and Technicians did not have full privileges.
In 1964, the FCC and the American Radio Relay League (ARRL) developed a program known as "Incentive Licensing," which rearranged the HF spectrum privileges. The General/Conditional and Advanced portions of the HF bands were reduced, with the spectrum reassigned to those in the Advanced and Amateur Extra classes. It was hoped that these special portions of the radio spectrum would provide an incentive for hams to increase their knowledge and skills, creating a larger pool of experts to lead the Space Age.[citation needed] It did not take effect until 1968.
Not exactly.
The changes known as "incentive licensing" were first discussed in 1963, when the first proposals were sent to FCC. There were at least 10 different proposals from groups and individuals, and the whole thing was discussed for years until the final decision was made in 1966. It wasn't just FCC and the ARRL involved.
Existing Advanced, General and Conditional licensees lost some privileges on 80/75, 40, 20, and 15. Most of the lost privileges were 'phone, but the bottom 25 kHz of the CW/digital part was Extra-only as well.
I remember back in 1976, when i became a Novice, there were some senior Advanced Class hams who were still somewhat bitter over the fact that they had once been Amateur Extra First Grade licensees with some exclusive privileges back in the day, only to see Generals eventually be given full privileges in 1951, and then to have some of their phone frequency privileges taken away and given only to the new-fangled Extras in 1968... may not seem like a big deal to us today, but it clearly was to some of them ...
The full-privileges-to-Generals thing happened in 1953, not 1951, and the Extra wasn't a new license in 1968.
see: https://en.wikipedia.org/wiki/Amateur_radio_licensing_in_the_United_States
I will check it - if that's where you got the info, some of it is in error.