Front side Diocletian
In order to solve the problem of succession, and to answer the question of who would be Emperor of the newly divided East and West, Diocletian created what has become known as the system of "tetrarchy", or "rule of four", whereby a senior emperor would rule in the East and another senior emperor would rule the West, and each would have a junior emperor. Among the many titles traditionally bestowed on Roman emperors, the most important was that of Augustus and therefore only the two senior emperors took this title, with the junior emperors receiving the lesser title of Caesar. Diocletian intended that when the senior emperor retired or died, the Caesar would take his place and choose a new junior emperor Caesar, thus solving the problem of succession. That is why the emperor is depicted twice on the 50 SC banknote. And because of the monetary reforms Diocletian took, as a sign of his actions, another coin is depicted next to his bust.
Back side Diocletian Palace
At the end of the third century AD, the Roman Emperor Diocletian built his palace on the bay of Aspalathos. Here, after abdicating on the first of May in A.D. 305, he spent the last years of his life in peace, mostly enjoing in his garden. The bay is located on the south side of a short peninsula running out from the Dalmatian coast into the Adriatic, four miles from the site of Salona, the capital of the Roman province of Dalmatia. The Palace is built of white local limestone of high quality, most of which was from quarries on the island of Brac. It is the same quality stone from which the White House in Washington was built of.
The Palace is one of the most famous and integral architectural and cultural constructs on the Adriatic coast and holds an outstanding place in the Mediterranean, European and world heritage, and that is why in 1979 UNESCO, in line with the international convention concerning the cultural and natural heritage, adopted a proposal that the historic Split inner city, built around the Palace, should be included in the register of the World Cultural Heritage.
Quid pro quo - "Something for something" indicates a more-or-less equal exchange or substitution of goods or services.