Is it forbidden to call grandparents by their first name?

Question

I saw in the biography of R Shimshon Dovid Pincus references to "Zaidy Mann and Zaidy Pincus" and this got me wondering whether there was a prohibition to call grandparents by their first names? I know some families have a tradition to call each set of grandparents by different names (e.g., Zaidy & Saba) although this could be for mere convenience.

I know there is a prohibition to call one's father and mother by their first name (YD 240:2, see details here). Does this extend further? This suggests it doesn't apply to parents-in-law so what about grandparents?

Answer 1

The rema in yore deah 240:22 holds that kibud av also applies to אבי אביו- the father of his father.

The sefer פסקים ותשובות (יו"ד סימן רמ סעיף סא) holds that this also applies to not calling the father of his father by his first name. In footnote 525 he brings a machlokes to the exact level the same halachos of kibbud of a father apply to the grandfather, but it is not clear of that would change the psak about calling a grandfather by his first name.

There might also be a difference between a father of a father and the father of a mother or the mother of a father/mother (the Rema only mentioned the first). Per the פסקים ותשובות many hold that all are included

פסקים ותשובות ע"י הרב אהרן ארי’ כ"ץ , ירושלים ה’ תשע"ז