Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Dark blue concrete wall with some small dust spots.
Source Atle Mo
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Remixed from a PNG that was uploaded to Pixabay by VictorianLady
Source Firkin
Mostly just mucked about with the colours and made one of the paths in the lead frame opaque. The glass remains transparent.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Same classic 45-degree pattern, dark version.
Source Luke McDonald
Prismatic Snowflakes Pattern 3 No Background
Source GDJ
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
From a design found in 'History of the Virginia Company of London; with letters to and from the first Colony, never before printed', Edward Neill, 1869.
Source Firkin
Thin lines, noise and texture creates this crisp dark denim pattern.
Source Marco Slooten