Upstairs Downstairs is a shawl in the shape of a right angle isosceles triangle of 100 x 200 cm (altitude x hypothenuse).
It is featuring Staircase Pattern, a stockinette slip stitch pattern (mosaic) from Barbara Walker’s “Treasuries”, in combination with garter stitch.
The shawl is worked in three colours.

You will need of
Colour A ca. 1.020 yds
Colour B ca. 550 yds
Colour C ca. 330 yds

Be sure to have enough contrast between the chosen main colour and the other two colours for the mosaic designs to arise effectively.
This pattern is appropriate for knitters with some experience with slip stitch patterns as well as basic knitting knowledge of k, p, m1l, m1r, kfb.
The instructions are written in detail and also contain charts for each section of the project.

Many thanks to the testers for their wonderful knits:

CablingKaren
nickknitterton - who made a special four coloured version!
isla
veron

And thanks to Yilla, evlupps and Palacsinta for their useful hints!