Question: How to combine `$\mathbb{C}$` with the letters `$A$,$L$,$M$` such that size of `\mathbb{C}` is of small size?[![Fig](https://i.stack.imgur.com/GUXsA.png)](https://i.stack.imgur.com/GUXsA.png)

Answer: 
```
\documentclass{article}
\usepackage{amsmath,amssymb}

\newcommand{\AC}{%
  \mathord{\vtop{%
    \ialign{%
      \hfil##\cr
      $A$\cr
      \noalign{\nointerlineskip}
      $\scriptscriptstyle\mathbb{C}\mkern2mu$\cr
    }%
  }}%
}
\newcommand{\LC}{%
  \mathord{\vtop{%
    \ialign{%
      \hfil##\cr
      $L$\cr
      \noalign{\nointerlineskip}
      $\scriptscriptstyle\mathbb{C}\mkern2mu$\cr
    }%
  }}%
}
\newcommand{\MC}{%
  \mathord{\vtop{%
    \ialign{%
      \hfil##\cr
      $M$\cr
      \noalign{\nointerlineskip}
      $\scriptscriptstyle\mathbb{C}\mkern6mu$\cr
    }%
  }}%
}

\begin{document}

$\AC\quad \LC \quad \MC$

\end{document}

```

[![enter image description here](https://i.stack.imgur.com/rPtJ6.png)](https://i.stack.imgur.com/rPtJ6.png)
While the OP refers to math letters A, L, and M, the image gives an impression of sans serif. Here is the macro `\subC[<fine adjustment>]{letter}`

```
\documentclass{article}
\usepackage{txfonts,stackengine}
\newcommand\subC[2][0pt]{%
  \ensurestackMath{\stackengine{-.03ex}{\mathsf{#2}\kern-\dimexpr#1\relax}{%
    \scriptscriptstyle\varmathbb{C}}{U}{r}{F}{F}{S}\kern#1}%
}
\begin{document}
$\subC{A}\quad \subC{L}\quad \subC[.5pt]{M}$
\end{document}

```

[![enter image description here](https://i.stack.imgur.com/zgLqC.jpg)](https://i.stack.imgur.com/zgLqC.jpg)

A version that caters to the normal serif letters:

```
\documentclass{article}
\usepackage{txfonts,stackengine}
\newcommand\subC[2][.3pt]{%
  \ensurestackMath{\stackengine{-.03ex}{#2\kern-\dimexpr#1\relax}{%
    \scriptscriptstyle\varmathbb{C}}{U}{r}{F}{F}{S}\kern#1}%
}
\begin{document}
$\subC{A}\quad \subC{L}\quad \subC[1.0pt]{M}$
\end{document}

```

[![enter image description here](https://i.stack.imgur.com/27q3m.jpg)](https://i.stack.imgur.com/27q3m.jpg)