Juxtaposition, multiplication, application, and composition

Juxtaposition is certainly a common, efficient way of combining two things.

I want to give here three separate examples of such.

1. Multiplication 

If $a$ and $b$ are two real numbers, we routinely write $ab$ to denote their (multiplicative) product.

If necessary, we put a centered dot between them to prevent ambiguity.

Thus, for example, $5\cdot 7 = 35$, while $57$ is just that.

2. Function application.

Suppose $x$ is a real number, and $f:\R\to\R$ is a real-valued function of one real variable.

It is convenient to write $xf$ for the value of $f$ at $x$.

Of course more conventional ways of writing this are $(x)f$ or $f(x)$, depending on whether you are letting functions operate to the right or left of their argument.

3. Composition 

Suppose we have three sets $X, Y$ and $Z$, 

and functions $f:X\to Y$ and $g:Y\to Z$.

We can then define a composite function $fg$ by $x(fg) = (xf)g$.