There are multiple internal forms of `x / 2`. I ran into the same problem here (with `1/4`): [Using Hold correctly with Simplify and ComplexityFunction][1] This appears to work in all cases: MakeBoxes[expr_ / 2 | Rational[1, 2] expr_, fmt_] := FractionBox[MakeBoxes[expr, fmt], "2"] (3 + Sin[t])/2 ![Mathematica graphics](https://i.sstatic.net/rS6T8.png) You can better see what is going on with `FullForm`: HoldForm @ FullForm[expr_/2] HoldForm @ FullForm[Rational[1, 2]*expr_] > Times[Pattern[expr,Blank[]],Power[2,-1]] > Times[Rational[1,2],Pattern[expr,Blank[]]] You need to cover both the `Power[2,-1]` and `Rational[1, 2]` cases with your pattern. [1]: https://mathematica.stackexchange.com/a/18149/121