Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Simple gray checkered lines, in light tones.
Source Radosław Rzepecki
Prismatic Curved Diamond Pattern 5 No Background
Source GDJ
Remixed from a drawing in 'Paul's Sister', Frances Peard, 1889.
Source Firkin
Prismatic Abstract Geometric Background 3
Source GDJ
Prismatic Triangular Seamless Pattern III With Background
Source GDJ
From a drawing in 'An Old Maid's Love. A Dutch tale told in English', Maarten Maartens, 1891.
Source Firkin
Embossed lines and squares with subtle highlights.
Source Alex Parker
This one could be the shirt of a golf player. Angled lines in different thicknesses.
Source Olivier Pineda
All good things come in threes, so I give you the third in my little concrete wall series.
Source Atle Mo
Colour version of the original pattern.
Source Firkin
This reminds me of Game Cube. A nice light 3D cube pattern.
Source Sander Ottens
A seamless background of warped stripes on paper.
Source V. Hartikainen
Dark, lines, noise, tactile. You get the drift.
Source Anatoli Nicolae
Remixed from a drawing in 'The March of Loyalty', Letitia MacClintock, 1884.
Source Firkin
A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
Source Firkin
This light yellow background pattern consists of an irregular pattern of spots. Here's a light background pattern with yellowish tint.
Source V. Hartikainen
A background pattern with a look of rough fabric.
Source V. Hartikainen
It was called Navy Blue, but I made it dark. You know, the way I like it.
Source Ethan Hamilton
The tile this is based on can be retrieved by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin