Windows 10 version history

<section begin=Lede />Windows 10 is an operating system developed by Microsoft. Microsoft described Windows 10 as an "operating system as a service" that would receive ongoing updates to its features and functionality, augmented with the ability for enterprise environments to receive non-critical updates at a slower pace or use long-term support milestones that will only receive critical updates, such as security patches, over their five-year lifespan of mainstream support. It was first released in July 2015. <section end=Lede /> 
Channels.
Windows 10 Insider Preview builds are delivered to Insiders in three different channels (previously "rings"). Insiders in the Dev Channel (previously "Fast Ring") receive updates prior to those in the Beta Channel (previously "Slow Ring"), but might experience more bugs and other issues. Insiders in the Release Preview Channel (previously "Release Preview Ring") do not receive updates until the version is almost available to the public, but are comparatively more stable.
PC version history.
Mainstream builds of Windows 10 are labeled "YYMM", with YY representing the two-digit year and MM representing the month of planned release (for example, version 1507 refers to builds which initially released in July 2015). Starting with version 20H2, Windows 10 release nomenclature changed from the year and month pattern to a year and half-year pattern (YYH1, YYH2).
Version 20H2 (October 2020 Update).
The Windows 10 October 2020 Update (codenamed "20H2") is the tenth major update to Windows 10 as the cumulative update to the May 2020 Update, and carries the build number 10.0.19042. The first preview was released to Insiders who opted in to Beta Channel on June 16, 2020. The update began rolling out on October 20, 2020. Notable changes in the October 2020 Update include:
Version 21H1 (May 2021 Update).
The Windows 10 May 2021 Update (codenamed "21H1") is the eleventh and current major update to Windows 10 as cumulative update to the October 2020 Update, and carries the build number 10.0.19043. The first preview was released to Insiders who opted in to Beta Channel on February 17, 2021. The update began rolling out on May 18, 2021. Notable changes in the May 2021 Update include:
Fast Ring / Dev Channel.
Fast Ring.
On December 16, 2019, Microsoft announced that Windows Insiders in the Fast Ring will receive builds directly from the RS_PRERELEASE branch, which are not matched to a specific Windows 10 release. The first build released under the new strategy, build 19536, was made available to Insiders on the same day.
The MN_RELEASE branch was available from May 13, 2020 to June 17, 2020. The branch was mandatory for all Windows Insiders.
Dev Channel.
As of June 15, 2020, Microsoft has introduced the "channels" model to its Windows Insider Program, succeeding its "ring" model. All future builds starting from build 10.0.20150, therefore, would be released to Windows Insiders in the Dev Channel.
The FE_RELEASE branch was available from October 29, 2020 to January 6, 2021. The branch was mandatory for all Windows Insiders until December 10. Afterward, Windows Insiders could choose to move back to the RS_PRERELEASE branch.
The CO_RELEASE branch has been available since April 5, 2021. The branch is mandatory for all Windows Insiders.