Watch dials with numeric hour markers use either Arabic numerals or Roman numerals. The choice is a strong design-language signal: each convention carries cultural and aesthetic associations that trained collectors read instantly. Arabic numerals are maximum legibility at distance: a "3" reads as 3 in milliseconds at any orientation; a "III" requires a fraction-of-a-second pattern-match. This is why tool watches (pilot, dive, military) almost universally use Arabic numerals.
Roman numerals read as traditional / formal / dress. The convention dates to clock-making: church-tower clocks, grandfather clocks, and traditional pocket watches used Roman numerals; the visual association with traditional Western timekeeping remains strong. Modern Roman-numeral dials read as continuous with this tradition; modern Arabic-numeral dials read as functional / modern / instrument. The choice is rarely about literal legibility; it is about cultural positioning.
"IIII is wrong arithmetic and right design. The dial would be lopsided otherwise."- Watch designer on the IIII-vs-IV convention
A persistent convention on Roman-numeral dials: the "4" position uses IIII rather than IV. Standard Roman-numeral notation is subtractive: 4 = IV (one before five). Watch dials use the additive form IIII (four I's) for purely visual reasons: the eight-letter VIII at the opposite (8) position is visually balanced by a four-letter IIII rather than a two-letter IV. The resulting dial has more symmetric visual weight distributed across the 12 hour positions. Almost every modern dress watch with Roman numerals uses IIII: Cartier Tank, Breguet Classique, Vacheron Patrimony, Patek Calatrava with Roman numerals.
A small number of modern designs deliberately use IV for modernist or design-statement reasons: some Bauhaus-influenced microbrands, a handful of haute-horlogerie pieces with explicit modernist intent. The IV convention reads as "technically correct but visually unbalanced"; collectors generally consider IIII the proper convention for traditional Roman-numeral dials.
Mixed numeral systems are rare but exist. Some Cartier references use Arabic at 12 and Roman at other positions; some heritage chronograph dials use Arabic at cardinal hours (12, 3, 6, 9) and either no markers or applied indices at other positions. These are deliberate design choices for visual rhythm rather than convention errors. The default is full-Roman or full-Arabic across the entire 12-hour scale; mixed numeral systems require explicit design justification.