Clock faces that are labeled using Roman numerals conventionally show IIII for four o'clock and IX for nine o'clock, using the subtractive principle in one case and not the other. There are many suggested explanations for this, several of which may be true:
Louis XIV, king of France, who preferred IIII over IV, ordered his clockmakers to produce clocks with IIII and not IV, and thus it has remained.
Using the standard numerals, two sets of figures would be similar and therefore confusable by children and others unused to reading clockfaces: IV and the VI; and IX and XI. Since the first pair are additionally upside down on the face, an added level of confusion would be introduced. It is used to make greater character distinction between them by using IIII and VI
The four-character form IIII creates a visual symmetry with the VIII on the other side, which the two-character IV would not.
With IIII, the number of symbols on the clock totals twenty Is, four Vs, and four Xs, so clock makers need only a single mold with a V, five Is, and an X in order to make the correct number of numerals for their clocks: VIIIIIX. This is cast four times for each clock and the twelve required numerals are separated:
V IIII IX
VI II IIX
VII III X
VIII I IX
The IIX and one of the IXs are rotated 180° to form XI and XII. The alternative with IV uses seventeen Is, five Vs, and four Xs, requiring the clock maker to have several different molds.
Only the I symbol would be seen in the first four hours of the clock, the V symbol would only appear in the next four hours, and the X symbol only in the last four hours. This would add to the clock's radial symmetry.
Many clocks use IIII because that was the tradition established by the earliest surviving clock, the Wells Cathedral clock built between 1386 and 1392. It used IIII because that was the typical method used to denote 4 in contemporary manuscripts (as iiij or iiii). That clock had an asymmetrical 24-hour dial and used Arabic numerals for a minute dial and a moon dial, so theories depending on a symmetrical 12-hour clock face do not apply.