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The gold solidus (pl. solidi; lit. 'solid') was a gold coin first issued by the Romans by around 301 AD, and later, from 312 AD onwards. Originally a coin that was first issued by Diocletian (301 AD), it was a gold coin weighing about 5.5 grams during its first production and composed of 24 carats and was initially worth 1, 000 denarii.  Diocletian's solidi were the first types of coins to be issued with the name, although their production was only limited and was eventually discontinued in favor of the more popular aureus.
The reintroduction of the solidus began with the fall of the Roman standard gold coin – the aureus, which was by then far too expensive when compared to the more common but extremely debased silver currency that was used all throughout the Roman Empire. With the discontinuation of the aureus, the solidus soon began to replace it under the edict of Constantine the Great in 312 AD. All production of Roman aurei ceased, until by 315 AD the solidus wholly replaced the aureus as the standard gold coin of the Roman Empire. Constantine's solidus weighed lesser than Diocletian's original, at only 4.5 grams – however, due to the increasingly debased status of Roman currency by his time, Constantine's solidi were now worth 275, 000 denarii. Unlike aurei which were strictly used for private transactions, investments, and the payment of taxes, solidi were also used in the general market albeit in very limited circumstances. Just like its predecessor the aureus, solidi were strictly off-limits to commoners and the production of solidi were restricted to specialized minters in the confidence of the emperor, unlike base metal coins which had mints of their own both within the Roman Empire proper and its subsequent states. 
During the height of the Byzantine Empire's influence, the solidus was also adopted as its standard gold coin, then taking on the name of nomismata (lit. 'currency') to mean that solidi were the de facto currency of the Byzantine Empire.  As with Rome, the production of solidi as well as their use was supposedly limited only to Byzantium, although due to the ever expanding growth of trade and commerce, some solidi were even used outside of Byzantium, and even some examples from Rome proper itself had been circulated (in highly worn forms) throughout the whole of the Roman Empire and its states and small principalities. It has been reputed that solidi were even used by the Roman Empire to pay high ranking soldiers, or even common ones – and it is from this practice that the modern term 'soldier' was said to have originated. 
Solidi that circulated outside of Byzantium and Rome only occurred in limited quantities, and were either melted for their gold, or were extremely worn due to its relative softness, as the limited circulation of solidi did not only serve to preserve Rome's gold, but to allow worn solidi to be easily melted down and re-minted. Some examples of solidi circulated as far as Arabia and some areas of the Levant, where it inspired the creation of their own 'solidus' – the dinar. Since solidi became standard currency, lesser fractions of the coin were developed such as the tremissis and the semissis (one third solidus, and half-solidus respectively). Further evolution of the solidi took place when another type of gold coin was introduced in Byzantium by around 960 AD. At around this time, the Byzantine solidi had to undergo a total but gradual revamp and were then referred to as the histamenon.  Due to its retaining its original gold purity, the now revamped histamenon also had to undergo a physical change, becoming a slightly cup-shaped (scyphate) coin that deviated from its originally flat form. The now renamed solidus continued to be produced alongside its lesser twin the tetarteron until inevitable debasement caught up with both. By the time of Byzantine Emperor Alexios I Komnenos, a new type of gold currency would replace the solidus / histamenon and its twin – the hyperpyron.
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