Suppose ---- $x_1=y$ \scriptspace=10pt $x_1=y$ \bye ----- When we use pdftex then we get expected result: the first formula has default script space and the second has big script space. But when we use luatex then both formulae have default script space. When the first formula is removed then the resting one has expected script space 10pt with luatex too. I am unable to understand the logic of luatex behavior.  This is LuaTeX, Version 1.15.0 (TeX Live 2022) Regards Petr Olsak