I have a confession to make. I used to put two spaces after each
sentence, as it used to be done by typewriter typists before computers
[citation needed].

The moment when I started to form this habit was around 2010. I was
young and easy to impress, and I was picking up more advanced Emacs
use working on the TeX source for my diploma thesis. While editing
these large chunks of free-form text was when I discovered the
`fill-paragraph` function, otherwise known as `M-q`, for automatically
breaking text at the correct column width. This is very useful for
editing text files, so it quickly became a habit to press `M-q` often.
I'm still using it right now, in fact.

Now after a while of using this feature, I started to observe that
this didn't work exactly how I thought. Readjusting text where one
line ends with a full stop, it would surprisingly add two spaces in
the middle after breaking.

This text:

```text
Hello world.
This is a text.
```

Would turn into that text after a press of `M-q`:

```text
Hello world.  This is a text.
```

Then somewhere in Emacs documentation, I found key bindings jump back
and forth between sentences (`M-a`, `M-e`), and these also rely on
double-spaces to tell apart sentence ends from abbreviations (e.g. as
in Dr. Seuss).

I did not end up picking up these shortcuts, but it was enough for me
to rationalize the double spaces. In reality of course, I only started
with that habit so that `M-q` would not break the otherwise consistent
spacing style accidentally.

I used this for about 10 years now, and of course it didn't elude me
that I was the only one doing that, but it's hard to put down old
habits.

Recently, I found what I think is the remedy - `M-q` is actually
configurable in Emacs, and the old habit of pressing `M-q` will now
just fix my sentences to single-spaces again going forward.

This is what is needed:

```elisp
(setq sentence-end-double-space nil)
```

What do we learn from this story? I'm not sure. Maybe that old habits
are hard to shake. Some quirks creep in without you noticing and a
tiny change might fix them.

`M-q`