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I discovered yesterday that the lmodern font has some size problems with maths symbols (most notably the parenthesis and integral signs), especially when the font size used is 11pt . Loading that font with a nomath option solves this:
\usepackage[nomath]{lmodern}

I was using lmodern in almost all ...

Here's a not so MWE to show my problem and why I use the macros defined above to solve it after a first compilation:
\documentclass[11pt,letterpaper,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french]{babel}
\usepackage{lmodern}
\usepackage{textcomp}
\usepackage ...

I do use the amsmath package (and about a dozen of other maths packages as well). I don't understand the uses of \abovedisplayshortskip . What does it do?

I need all my equations to have a fixed constant space between the text above and under them. I hate variable spaces around equations. In my ...

I've found this solution, using \begingroup ... \endgroup , instead of using the macros defined above:
\begingroup\abovedisplayskip=0pt
\begin{equation}
y = x
\end{equation}
\endgroup

But then, how can I create a simple macro to simplify this solution, instead of typing all the extra code each ...

The following problem is pretty simple (if not straightforward), and I don't think it needs a MWE.

In a document, I defined two macros like this:
\newcommand*{\topless}{\setlength{\abovedisplayskip}{0pt}}
\newcommand*{\topmore}{\setlength{\abovedisplayskip}{11pt}}

The first macro is used to ...

Johannes,

what I'm asking is that all the equations in a given math environment (gather and align) are equaly spaced vertically. I want to get this equal vertical spacing for all align and gather environments. If an equation is really bigger than some other, I'll put it in a separate environment ...

Here's a MWE showing my troubles:

\documentclass[11pt,letterpaper,twoside]{book}
\usepackage{lmodern}
\usepackage{amsmath}
%\setlength{\jot}{\baselineskip}

\begin{document}

Blabla:
\begin{gather}
x^2 - \frac{1}{2} \, x = 3, \\
\frac{a^2}{b^3} \, y + 2 x \, y = y^2, \\
a - b = c.
\end ...

Well, I now have another point of view that I would like to try.

Before, I was using fixed spaces like \\[6pt] , \\[12pt] , \\[18pt] and even \\[24pt] (depending on the size and "type" of equations), for equations in a 12pts font document .

I hate it when LaTeX change the spacements to adapt ...

I guess you mean \setlength{\jot}{12pt} ?

Well, that's not very specific. I'm already asking how to do this properly!

Do you have a suggestion code? I tried using \\, but the spacing is way to short (and variable). Using \\ \\ gives some spacing troubles too.