Since WordPress 3.0, released in June 2010, the themes bundled with WordPress core have been named twentysomething, starting from twentyten.
The latest is Twenty Twenty-One ( twentytwentyone ), released with WordPress 5.6, ready for 2021.
2010 
2011 
2012 
2013 
2014 
2015 
2016 
2017 
2019 
2020 
2021
Here’s a table of the themes bundled into WordPress core by version.
| WordPress | 2021 | 2020 | 2019 | 2017 | 2016 | 2015 | 2014 | 2013 | 2012 | 2011 | 2010 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5.6 | ||||||||||||
| 5.5 | ||||||||||||
| 5.4 | ||||||||||||
| 5.3 | ||||||||||||
| 5.2 | ||||||||||||
| 5.1 | ||||||||||||
| 5.0 | ||||||||||||
| 4.9 | ||||||||||||
| 4.8 | ||||||||||||
| 4.7 | ||||||||||||
| 4.6 | ||||||||||||
| 4.5 | ||||||||||||
| 4.4 | ||||||||||||
| 4.3 | ||||||||||||
| 4.2 | ||||||||||||
| 4.1 | ||||||||||||
| 4.0 | ||||||||||||
| 3.9 | ||||||||||||
| 3.8 | ||||||||||||
| 3.7 | ||||||||||||
| 3.6 | ||||||||||||
| 3.5 | ||||||||||||
| 3.4 | ||||||||||||
| 3.3 | ||||||||||||
| 3.2 | ||||||||||||
| 3.1 | ||||||||||||
| 3.0 |
Note: There wasn’t a default theme for 2018.
I’ve updated my dynamic API reference for the themes that are delivered in WordPress core. See wp-a2z.org.
Note: The main SVN repositories contain all of the above themes.
For more info on WordPress source see wordpress.org/download/source