Turning a multi-speed bicycle into a single-speed one - basics explained

What to pay attention to (and check/consider) when modifying a multi-speed bicycle into a single-speed one.

Note: I forgot to mention this, but in a post I wrote there is a picture of dropout shapes that are convenient for single speed bicycles (for easily setting up optimal chain tension) - mostly horizontal dropouts:
https://bike.bikegremlin.com/158/bicycle-rear-hub/#2

Also, in the linked article, there are some solutions for the frames with vertical dropouts.

- Contents -
00:00 Introduction
01:01 Potential problems - things to consider
01:57 Chainline explained
03:56 Rear dropouts spacing (width)
05:30 How to change rear dropout spacing
07:01 Aligning the dropouts, after re-spacing them
07:41 Limits of dropout spacing change
09:08 Wheel adjustment

- Useful links -

What is a bicycle chainline, how to measure and adjust it:
https://bike.bikegremlin.com/1755/bicycle-chainline/

Rear wheel axle adjustment (centering) explained:
https://bike.bikegremlin.com/1461/hub-overhaul/#7

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