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The tetarteron (lit. 'quarter coin') was a name that referred to two types of Byzantian coins – the first being a gold coin that circulated from 960 AD to 1092 AD, and the second being a copper one that replaced the gold coin from 1092 on towards the middle of the 13th century. The tetarteron first began circulation solely as a gold coin of the Byzantine Empire, created contrary to the then standard solidus first issued by Constantine I to replace the older Roman aureus.  This remained so since 309 AD until the solidus experienced a type of competition due to the introduction of the tetarteron some six hundred years later by the emperor Nikephoros II Phokas. While similar in appearance and design to the original solidus (now referred to as the histamenon to differentiate it from the recently introduced tetarteron) it was actually two carats lighter than the original solidus that it intended to emulate. Later on the tetarteron also became smaller than the solidus / histamenon, and the two took on distinctly disparate looks that would later distinguish the two.
While 2 carats lesser than the histamenon, by edict of Nikephoros II its worth was supposed to be equal to that of the original currency. Although issuing two gold coins of the same worth, with (at first) very little distinctive differences seems to serve a very pointless purpose, some historians argue that the despite the purposes of issuing the tetarteron being unclear, it could be hypothesized that the use of the tetarteron was to increase the state revenues of the Byzantine Empire. Because histamena were numerous, it would have been impossible for Nikephoros II to replace it with the less valuable tetarteron, unless a total recall of all Byzantine gold currency be accomplished – which he did not commit to. Nikephoros II instead opted that taxes be paid using histamena, while state expenses would be dealt with using the less valuable tetartera, which was only equal to the histamenon by official edict only.  Some historians even suggest that the tetarteron was a type of gold coin that was to be issued in recently conquered areas in the eastern part of Byzantium, although why this needed to be done still remains an enigma. Whatever the reason for the creation of the tetarteron, its first run of production only took on a limited pace and it was not until the middle of the 11th century that the number of tetarteron produced by Byzantium grew close to the number of histamena. It is unclear whether the production of histamena were discontinued, or whether both tetartera and histamena were produced in consistently rising numbers until the eventual debasement of both currencies in the latter decades of the Byzantine Empire. 
The gold histamenon and tetarteron first began as twins in form, with a difference only in the weight of both coins (with the tetartera being lighter than the histamena). Later on, these changes would become more pronounced as histamena evolved to become more concave in shape, with decidedly wider borders and deeper 'wells' that made the coins resemble tiny cups (scyphate). While the need to change the look of the histamenon arose not only due to a need for distinction between the two coins, but for a need to reinforce the softer 24-carat histamena against wear and tear that could easily deface it due to its softer nature – the lighter, less pure tetarteron remained a flat coin, albeit becoming a thicker, smaller one.
The gold content of both the tetarteron and the histamenon soon underwent debasement in the same vein as their common ancestor the aureus, so that by the time of Michael IV the Paphlagonian, the histamenon (now measuring 25mm in diameter, veering form the original 20mm solidus of Contantine I) soon lost much of its purity, followed shortly by the tetarteron's subsequent debasement. When Alexios I Komnenos reformed the imperial coinage into what would be called the Komnenian money system, the histamena were replaced by the revamped gold hyperpyron.  The minting of histamena ceased, although the production of tetarteron continued. However, due to the new coinage system, the tetarteron ceased to be a gold coin and began to be minted in copper from 1092 onwards. This copper version issued under Alexios I began to replace the older follis copper coinage, although it retained its name in shortened form – as the tarteron. This copper issue of the tetartera too was discontinued by the latter part of the 1200s, to be replaced by another copper coin – the assaria. After the debasement of the last Byzantine gold coin, the hyperpyron, all subsequent mintings of gold coinage came to a drastic halt, spelling the end of the gold coins of Byzantium.
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