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?