What does 'horizontal extension' mean in Unicode proposal docs?

[Ken Whistler]

Take a look at the CJK unified code charts on the site:

https://www.unicode.org/charts/

The CJK unified ideographs are presented in a multi-column format, where 
each column represents a different source for the ideograph (and often 
shows a slightly different glyph appropriate, e.g., to a Chinese font 
versus a Japanese font, etc.).

A *horizontal* extension for CJK represents the addition of a new 
*source* (and corresponding glyph) where one was not present before.

If you look, for example, at the recently encoded CJK Extension I, all 
of the characters have a single (Chinese) source, because this 
repertoire was all added for recent additions in an important Chinese 
standard. However, in the future, if any of those characters also 
happens to be added to some Japanese, Taiwan, Korean, or other standard, 
a second source may be added for that character, to indicate its 
presence in something other than the original Chinese standard. That 
would be a horizontal extension. It wouldn't add a new CJK unified 
ideograph with a new code point -- instead, it would just add another 
source for an existing character.

A *vertical* extension just means the addition of more CJK unified 
ideographs. Thus the encoding of CJK Extension I in Unicode 15.1 was a 
vertical extension, adding 622 more CJK unified ideographs to the standard.

--Ken

On 11/13/2023 3:12 PM, Sławomir Osipiuk via Unicode wrote:
> The phrase 'horizontal extension' appears in many proposals related to 
> CJK characters, but isn't defined, and trying to intuit a definition 
> from context is a bit risky.
>
> What does this phrase mean formally, and what would the obvious 
> contrast 'vertical extension' mean?
>