Indices are the hour markers applied or printed on the dial to indicate the positions of the hours. The distinction that really matters is between printed and applied indices: the former are pigment deposited on the dial surface, the latter are separate metal elements fixed with welding or technical adhesive. From my bench I see every day how much this difference counts over time: an applied index keeps its defined edges for decades, a printed one wears away with the dial.
The shapes are many and each carries with it a precise design history. Simple rectangular batons are the zero point of instrumental design. Oblong indices, trapezoid or bathtub — the latter with long sides beveled and concave — belong to the Art Deco vocabulary and the dive watch era of the Sixties-Seventies. Faceted versions, with surfaces cut into multiple planes, catch light from different angles and create depth without strong colors: you find them on the Tissot Classic Dream as on the Seiko Presage Style60s, with opposite aesthetic logics but the same optical principle. Breguet numerals — Roman numerals of classical cut, attributed to Abraham-Louis Breguet — define a pocket watch aesthetic that has survived on wrist watch dials for more than two centuries.
Luminescence is an integral part of the history of indices, not an accessory. Before around 1960, radium paint was used: radioactive, effective, and today a serious problem on any vintage that wants to open. Then came tritium, then modern photoluminescent compounds. Today the reference is Super-LumiNova in various versions — BGW9, C1, C3 — with emissions ranging from white to green to blue. Rolex calls its own compound Chromalight, Seiko uses LumiBrite, Farer developed Lumicast, a ceramic-Super-LumiNova composite that produces applied indices more robust than traditional liquid lume. The charge color and the emission color in the dark can differ: something worth checking before choosing.
A technical case that has always interested me: on the Alterum Worldtimer the indices are printed on the underside of the sapphire crystal, not on the dial. The resulting visual depth is real, not a surface finish. It's a rare solution but consistent with a minimalist logic where every layer works.
On vintage watches that pass through my workshop, the condition of the indices is often the first indicator of a refinished dial: color, height relative to the surface, type of adhesive visible at the edges. An original index has a history that reads under magnification.