-ex

-ex, -ix

Naturalized Latin nouns ending in -ex and -ix vary in their plural forms, sometimes (as English words) adding -es and sometimes (as Latin words) changing the ending to -ices. The following table lists the most important words and their plural forms.

singular / principal meaning / plural

apex / highest point / apexes

/ (technical) / apices

appendix / supplement in book / appendices

/ (medical) / appendixes

codex / manuscript in book form / codices

cortex / outer part of an organ / cortices

duplex / apartment on two floors (AmE) / duplexes

helix / spiral curve / helices

ilex / tree or shrub / ilexes

index / list in book / indexes

/ (technical) / indices

matrix / mould, grid structure, etc. / matrices or matrixes

murex / mollusc / murices

radix / number or symbol / radices

silex / quartz or flint / no plural

simplex / thing in its simplest form / simplexes

vertex / highest point / vertices or vertexes

vortex / mass of whirling fluid / vortices or vortexes