The follis (Latin follis - plural folles) was a great bronze coin introduced around in 294 with the monetary reform of Diocletian. Its weight was around 10 grams and it contained ca. 4% of silver, mainly in the thin superficial layer. The word follis means small bag of coins and there are evidences that this term in the antiquity was used to define a pouch sealed to contain a fixed quantity of money.
The follis of Diocleziano, despite the efforts to freeze the cost of living through the edict about the maximum prices imposed in 301, was heavily devalued. At the time of Costantino I, the follis was smaller, 3,41g at the beginning and 1,70g later on, and with a very small amount of silver.
The fals (corruption of follis) was a bronze coin used in the Umayyad and Abbasid Caliphate since the end of the VII century, initially as imitations of the Byzantine folles.