Found it from an old email of Wolfgang's in the archive - not sure why I didn't see it before. Solution is to use: \definefontfeature[latin-smallcaps][smallcaps][script=latn] \definefontsynonym [Squar] [name:SquarishSansCT-Regular] [features=latin-smallcaps] instead of the first two lines in my example below.

On 20 Sep 2021, at 10:53, Bruce Horrocks wrote:

I'm trying to use smallcaps with a font called SquarishSansCT.

According to mtxrun it has smcp as a feature...

$ mtxrun --script font --list --info --pattern=squar [...] mtx-fonts | gsub features: mtx-fonts | mtx-fonts | feature script languages mtx-fonts | mtx-fonts | c2sc grek dflt mtx-fonts | latn dflt mtx-fonts | smcp grek dflt mtx-fonts | latn dflt mtx-fonts |

...but I can't seem to get that feature to work in this MWE which works for TexGyreSchola.

\definefontfeature [smallcaps] [always] [smcp=yes] \definefontsynonym [Squar] [name:SquarishSansCT-Regular] [features=smallcaps] \definefontsynonym [Schola] [name:texgyreschola-regular] [features=smallcaps]

\definefont [chapterTitle] [Squar at 18pt] \definefont [anotherTitle] [Schola at 18pt]

\starttext {\chapterTitle A Chapter Title} \par {\anotherTitle A Chapter Title} \stoptext

Have I missed something or is it the font? — Bruce Horrocks Hampshire, UK

— Bruce Horrocks Hampshire, UK