Plastics technology

The fifth entry is o índice de fluidez, which English calls « the melt flow index ». O índice has exactly one plural, os índices, and there the question closes. « Index » has two, and it is the sense, not the taste of the writer, that decides between them. « Indexes » belongs to the list at the back of a book and to the ones a database keeps. « Indices » belongs to a number that stands for a ratio or a measurement: price indices, refractive indices, the melt flow indices of four grades of the same polymer. A materials report that says « melt flow indexes » is understood, and still reads as the work of somebody outside the trade. The same habit reaches a small family of nouns that came in from Latin and kept their old endings for scientific work: « formula » gives « formulae » in a laboratory and « formulas » in ordinary prose, and « matrix » gives « matrices » wherever mathematics is meant. The short rule is that the technical sense holds on to the Latin plural and the everyday sense lets it go.