Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Prismatic Abstract Geometric Background 3
Source GDJ
Prismatic Groovy Concentric Background 3 No Black
Source GDJ
Derived from an image that was uploaded to Pixabay by Darkmoon1968
Source Firkin
Just the symbols of the signs of the zodiac distributed in a chequer board-like pattern
Source Firkin
This yellow background consists of a pattern of glossy gold buttons arranged in polka dot style on a seamless texture. Here's a pale yellow background pattern. Feel free to use it for your needs!
Source V. Hartikainen
Alternative colour scheme. Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Derived from a PNG that was uploaded to Pixabay by nutkitten
Source Firkin
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
The classic 45-degree diagonal line pattern, done right.
Source Jorick van Hees
It’s big, it’s gradient—and it’s square.
Source Brankic1979
A nice and simple white rotated tile pattern.
Source Another One
The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Prismatic Curved Diamond Pattern 8 No Background
Source GDJ
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
A fun-looking elastoplast/band-aid pattern. A hint of orange tone in this one.
Source Josh Green
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Zero CC tileable bark texture, photographed and made by me. CC0
Source Sojan Janso