The Half-Follis denomination
The half-Follis was a Roman Imperial and Byzantine denomination valued at, well, half a follis.

It was struck only infrequently, mostly in the years the Diocletian coin reform in 294 AD and before the shrinking follis made the half-follis impractical.

When the Follis was reintroduced by Anastasius in 498, the half-follis was also reintroduced. It was valued at 20 nummi and marked with a large 'K' for 20.
Half-Follis

(1) Claudius II 318-319 AD
Obverse: veiled laureate head right; DIVO CLAVDIO OPTIMO IMP
Reverse: emperor seated left on curule chair, holding short scepter, raising right hand; REQVIES OPTIM_ORVM MERITORVM / •TS•A•
Ref: RIC VII Thessalonica 26
(2) Maximian 318-319 AD
Obverse: Veiled laureate head right; DIVO MAXIMIANO SEN FORT IMP
Reverse: Emperor seated left on curule chair, holding short scepter, raising right hand; REQVIES OPTIM_ORVM MERITORVM / SIS
Ref: RIC VII Siscia 41
(3) Romulus 309-312 AD
Obverse: DIVO ROMVLO N V BIS CONS, Bare head of Romulus facing right
Reverse: AETERNAE MEMORIAE, Domed shrine with one door open, eagle standing facing right on roof; MOSTP in exergue
Ref: RIC VI Ostia 59