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) Maurice Tiberius 593 AD
Obverse: D N MAVR TIBER PP AV. Helmeted and cuirassed bust facing, holding globus cruciger and shield.
Reverse: Large mark of value K; ANNO to left, cross above, X/I to right, A in exergue.
Ref: SBV 497; DOC 56a