Question:
Is Al naarot bavel said after mayim achoronim, or before?
Answer:
Al naarot bavel and Shir Hamaalos are said before washing mayim achronim. The reason is because we should preferably not have any separating (hefsek) between mayim achronim and bentching.
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Sources:
Aruch Hashulchan 181-9, Ketzos Hashulchan 43-1. According to some Rishonim ( Mordechai Brachos 204, Tosefta Brachos 5-14) what he gemora says “Tikef l’nitila bracha” means that washing mayim achronim has to be immediately before bentching. Maharshal 34, Taz O:CH 166-2.