I remember being taught in history classes to abbreviate century by writing a large capital C followed by the ordinal number as in:
C18th without the full-stop (period).
Recently I have noticed on ELU (it wasn't something I paid particular attention in the past) the lower-case c after a cardinal number and with a period, as in:
18c without the period. Would either
18th c. or
C18 be considered incorrect?
I checked online and The Oxford English Dictionary says century is abbreviated thus: c. but no indication if the number preceding should be ordinal or cardinal. On the first page of Google results I also found the Monash University recommended symbol, but without th.
In The Chambers Dictionary 12th Edition, it lists both C and c as abbreviations for century:
C (preceding numeral, eg C21, twenty-first century)
c (following numeral, eg 21c, twenty-first century)
It also lists:
cen. abbrev: central; century
cent. abbrev: centigrade; central; centum (L), a hundred; century.
Why is the ordinal number e.g. 18th, 21st ignored? And finally, which is the most commonly accepted and recommended abbreviation?