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.NET Standard

A given .NET Standard defines (but does not implement) the APIs that are available across all .NET implementations that support this standard. The individual standards are version numbered (for example 1.6 or 2.0).
.NET Standard will replace Portable Class Libraries (PCLs) as the tooling story for building multi-platform .NET libraries.

1.6

2.0

.NET Standard 2.0 added over 20,000 APIs to 1.6. It is a strict superset of 1.6.
.NET Standard 2.0 only has 43 APIs that .NET Framework 4.6.1 doesn't support
.NET Standard 2.0 is supported by the following .NET implementations
.NET Standard 2.0 is included with the .NET Core 2.0 SDK and with Visual Studio 2017,
.NET Framework 4.8 implements .NET Standard 2.0 and won't move to 2.1

2.1

.NET Standard will be implemented (or is already implemented?) by .NET Core 3.0 as well as upcoming versions of Xamarin, Mono, and Unity.
2018-09-05: Announcing .NET Standard 2.1

See also

The .NET Standard metapackage NETStandard.Library.

Links

This interactive chart allows to choose a .NET Standard version and then displays which .NET implementation supports the chosen version.
This github repository contains the principles and definition of the .NET Standard.
Especially interesting is the src/netstandard/ref directory which contains *.cs files that define the APIs of a standard.

Index