How to make underbracket thinner

Question

Consider this code,
documentclass{article}
usepackage{mathtools}
begin{document}
[
C(s)=underbracket{frac{K_1}{s}}_{text{forced}}+underbracket{frac{K_2}{s+2}+frac{K_3}{s+4}+frac{K_4}{s+5}}_{text{natural}}
]
end{document}

and the result below

I think the underbracket is too thick. How to make it thinner?

Mico · Accepted Answer

Excerpting from p. 14 of the user guide of the mathtools package (highlighting added):

The user guide says that the default width of underbrackets is ca 5/18ex = ca. 0.28ex. The following screenshot shows expressions with underbrackets that involve the default width as well as one half, two thirds, and three quarters of the default width. Choose your favorite.

documentclass{article}
usepackage{mathtools} % for 'underbracket' macro
begin{document}
[
% default width: 5/18ex = ca 0.28ex
 underbracket{1+2+3}_{text{default}} % default width
+underbracket[0.140ex]{1+2+3}_{text{tubelike}}% (1/2)default
+underbracket[0.187ex]{1+2+3}_{text{skinny}}  % (2/3)default
+underbracket[0.210ex]{1+2+3}_{text{thin}}    % (3/4)default
+underbracket[0.280ex]{1+2+3}_{text{default}} %      default
]
end{document}

muzimuzhi Z · Answer

The thickness of brackets can be specified in the first optional argument of underbracket and overbracket. The default thickness is the height of $braceld$ (in current math font).
documentclass{article}
usepackage{mathtools}
begin{document}
[
  C(s) = 
  % use thickness 0.5pt
  underbracket[.5pt]{frac{K_1}{s}}_{text{forced}} + 
  % use default thickness
  underbracket{frac{K_2}{s+2}+frac{K_3}{s+4}+frac{K_4}{s+5}}_{text{natural}}
]
end{document}

You can also change the thickness of brackets once for all:
documentclass{article}
usepackage{mathtools}
usepackage{xpatch}

% change default thickness of brackets to .6pt
MHInternalSyntaxOn
xpatchcmdupbracketfill
  {sboxz@{$braceld$}edefl_MT_bracketheight_fdim{thehtz@}}
  {edefl_MT_bracketheight_fdim{.6pt}}
  {}{fail}

xpatchcmddownbracketfill
  {sboxz@{$braceld$}edefl_MT_bracketheight_fdim{thehtz@}}
  {edefl_MT_bracketheight_fdim{.6pt}}
  {}{fail}
MHInternalSyntaxOff

begin{document}
[
  C(s) = 
  underbracket{frac{K_1}{s}}_{text{forced}} + 
  % use default thickness
  overbracket{frac{K_2}{s+2}+frac{K_3}{s+4}+frac{K_4}{s+5}}^{text{natural}}
]
end{document}

How to make underbracket thinner

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