Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Inspired by an image that was uploaded to Pixabay by geralt
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Not sure if this is related to the Nami you get in Google image search, but hey, it’s nice!
Source Dertig Media
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Dark blue concrete wall with some small dust spots.
Source Atle Mo
Prismatic Geometric Pattern Background 2 No Black
Source GDJ
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Simple gray checkered lines, in light tones.
Source Radosław Rzepecki
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
Did some testing with Repper Pro tonight, and this gray mid-tone pattern came out.
Source Atle Mo
Based on several public domain drawings on Wikimedia Commons. This was formed from a rectangular tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
Prismatic Abstract Geometric Background 2 No Black
Source GDJ
Prismatic Curved Diamond Pattern 6 No Background
Source GDJ
These dots are already worn for you, so you don’t have to.
Source Matt McDaniel
A seamless pattern created from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
This was formed by distorting an image of a background on Pixabay.
Source Firkin
An attempt for cleaning up the original image in a few steps.
Source Lazur URH
Prismatic Polka Dots Mark II No Background
Source GDJ
A pattern drawn in Paint.net and vectorized in Vector Magic.
Source Firkin
The first pattern on here using opacity. Try it on a site with a colored background, or even using mixed colors.
Source Nathan Spady
The act or state of corrugating or of being corrugated, a wrinkle; fold; furrow; ridge.
Source Anna Litvinuk
A yellow tiled background... Blurriness, bokeh effect and rectangles pattern in one mix.
Source V. Hartikainen
Light gray grunge wall with a nice texture overlay.
Source Adam Anlauf