# Lower-case blackboard bold displays inconsistently

Lower-case blackboard bold (\mathbb) letters display inconsistently for different users: for me, they display (incorrectly, I assume) as plain Latin font. See example below. I'm on Chrome 68.0.3440.106 on Ubuntu Linux.

However, at least one other user sees the lower-case characters as expected, in blackboard bold. See this comment for an image.

Is this intended behavior? Why is there a discrepancy in how these characters display? I know that in LaTeX, \mathbb is not supported for lower-case letters (if I recall correctly), but it seems to be partially supported now in MathJax.

# Example

MathJax code:

\begin{align*}
&\mathbb{ABCDEFGHIJKLMNOPQRSTUVWXYZ} \\
&\mathbb{abcdefghijklmnopqrstuvwxyz} \\
\end{align*}


Rendered:

\begin{align*} &\mathbb{ABCDEFGHIJKLMNOPQRSTUVWXYZ} \\ &\mathbb{abcdefghijklmnopqrstuvwxyz} \\ \end{align*}

What I see: Only the capital letters and lower-case $k$ display as blackboard bold font. See the following image:

• P.S. I see the same thing on my mobile phone (Chrome on Android).
– 6005
Aug 16, 2018 at 21:01
• Upper case blackboard bold are really the only ones specified by Knuth. Plus $k$. So whoever added the extra lower case letters, depended on you to have to proper fonts. Aug 17, 2018 at 0:25
• I see the rendered text the same way @6005 sees it. I'm using Microsoft Edge 42.17134.1.0 on Windows 10.
– JRN
Aug 17, 2018 at 1:10

This is expected behavior, see the MathJax documentation on character fallbacks. The default (CM-derived) fonts do not have the necessary glyphs and MathJax's falback chain prefers available glyphs over system fonts (so that it has glyph metrics for its layout).

• So blackboard bold has only one lowercase glyph for $\Bbb k$, and the rest are just Roman?
– Asaf Karagila Mod
Aug 17, 2018 at 15:19
• Thanks for the answer. I'm still a bit puzzled that the behavior is intended. Why define k and not the other characters, when indeed on some systems all the other characters are defined? Why is it hard to port the same fonts to my system, or to integrate them in with computer modern?
– 6005
Aug 17, 2018 at 15:49
• @6005 This is a guess, but I suspect k exists to denote fields. Blackboard bold basically only exists for number systems and lowercase blackboard bold is not standard (not really necessary either).
– user29123
Aug 17, 2018 at 18:27
• Asaf: yes, that's correct (for the default fonts). @6005 MathJax needs glyph specific information for its layout. That's why it prefers to fall back to other glyphs it has information for rather than other fonts on the system (but it does fall back to those if all else fails). FWIW, adding better font options and custom fallback behavior is planned for MathJax v3. Aug 22, 2018 at 10:54
• @PeterKrautzberger Thank you for the response!
– 6005
Aug 22, 2018 at 14:32