* Mũthũrũ: This is the most common and general term for grandfather, regardless of whether it's the father's or mother's side.
* Mũthũrũ wa mũtheru: This translates to "grandfather from the father's side".
* Mũthũrũ wa mũnyanya: This translates to "grandfather from the mother's side".
So, depending on the context, you can choose the most appropriate term.