Until D-Day, the fatal day, when they had splintered apart and became two girls who couldn’t stand the sight of each other, and one who had disappeared into a madhouse.
-- The Narrator (Prologue)

Importance: This quote foreshadows the fallout between Mab, Osla, and Beth without giving away what exactly happened. It is clear that Osla feels guilty for something, and her separation from the other two women happened at a critical point in the war. This means that the reader is aware at what point in the novel things might go wrong, but can only guess what exactly causes their friendship to break. It also forces the reader to question if the woman who was sent to the sanatorium is a reliable narrator or not. All of this will be answered in the sections set in the past.

A good husband might have been the fastest way up the ladder toward safety and...
-- Mab (Chapter 1.)