Athens was the first proposed protocol-amendment for Tezos. Two proposals - Athens A and Athens B - were proposed by Nomadic Labs in February 2019.
Of the two proposals, Athens A sought to increase the gas limit and reduce the required roll size for baking from 10,000 tez to 8,000 tez. Athens B only sought to increase the gas limit. Athens A was voted with a Super-majority and was autonomously activated into the protocol in May 2019.
For a full list of changes, be sure to read this corresponding blog post from Nomadic Labs and reflections by Jacob Arluck.
Brest A was the first proposed amendment rejected during the Exploration Period. Submitted in June 2019, it received only 0.35% of the votes during the Proposal Period. But as it had no competition, the system promoted it. The amendment was then rejected in the Exploration Period with only 0.26% of favourable votes. The 80% Super-majority was not reached, neither was the minimum Quorum required to validate it.
This proposal would have fixed a security breach linked to the rehashing push during the Athens protocol change. Moreover, it would have facilitated the amendment's invoice tracking. But the invoice for this proposal, 8,000 tez, was much higher than the usual cost.
The Babylon proposal was composed of two proposals made in July/August 2019: Babylon and Babylon 2. Babylon 2 was built by Nomadic Labs, Cryptium Labs (Metastate), and Marigold, after receiving feedback on the first Babylon proposal. The teams proposed a new tweaked version in the same proposal period.
Notable changes included a new variant of the consensus algorithm (Emmy+). There were new Michelson features and accounts rehaul to aid smart contract developers. The accounts rehaul enabled a clearer distinction between "tz" and "KT" addresses. Furthermore, there was a refinement of the Quorum formula and the addition of the 5% threshold.
Babylon was autonomously activated into the protocol in October 2019.
For a full list of changes, be sure to read the corresponding blog posts from Nomadic Labs, and Cryptium Labs (Metastate).
Carthage was the first proposal to be rejected during the Proposal Period. Since the Babylon change, it now took a minimum of 5% approval to move to the Exploration Period and Carthage only obtained 3.5%.
The purpose of this proposal was to increase the gas limit per block and per operation by 30% to improve the accuracy of the existing formula used for calculating baking, endorsing rewards, and to fix various minor issues.
Carthage 2.0 was then proposed and overwhelmingly accepted in December 2019 partly due to the Nomadic Labs and Cryptium Labs (Metastate) contributions.
Notable changes included increasing the gas limit per block and per operation by 30%, improving the accuracy of the formula used to calculate baking and endorsing rewards, as well as several minor improvements to Michelson. The main difference with Carthage was the new and more secure formula to calculate rewards.
Carthage 2.0 was autonomously activated onto the protocol in March 2020.