Definition of "prefatory"

We must begin by showing how the Diamond first fell into the hands of my uncle Herncastle, when he was serving in India fifty years since. This prefatory narrative I have already got by me in the form of an old family paper, which relates the necessary particulars on the authority of an eye-witness.

1868 January 4 – June 6, [William] Wilkie Collins, “First Period. The Loss of the Diamond (1848). […]”, in The Moonstone. A Romance. […], volume I, London: Tinsley Brothers, […], published 1868, chapter I, pages 13–14

The "5th Mo. (May) 1939" issue of Bradshaw's Railway Shipping, and Hotel Guide for Great Britain and Ireland appears in a cream cover printed in blue and gold, and a prefatory note explains that, as 1939 is being celebrated as the centenary year of Bradshaw, it is intended to adopt this livery from now until the September issue inclusive. For October, the centenary month, Bradshaw will be published in a gold cover printed in red and blue.

1939 June, “Railway Literature: Bradshaw's Guide”, in Railway Magazine, page 465