Available Characters with iso-8859-1

Question

\documentclass[]{article}

\usepackage[iso-8859-1]{inputenx}
\usepackage[T1]{fontenc} 

\begin{document}
¡
£
§
©
ª
«   
¬   
®   
\end{document}

I was wondering. When I typeset a document using the package+option \usepackage[iso-8859-1]{inputenx} then I would naturally assume that I can use all characters that are defined for iso-8859-1 (Wikipedia link).

My example document here works but as soon as I add some characters then I get an error message. For example the Yen symbol (¥):

! LaTeX Error: Command \textyen unavailable in encoding T1.

In the inputenx documentation there you can find the following code in the iso-8859-1 section:

\ProvideTextCommandDefault{\textyen}{%
    \TextSymbolUnavailable\textyen
}

Maybe this is related.

On the other hand maybe the error nessage refers to the fontenc T1.

Main Question

Why are not all iso-8859-1 chatacters are availabe when using \usepackage[iso-8859-1]{inputenx}?

Answer 1

the input encoding specifies the interpretation of the input bytes, and there the byte is correctly interpreted as a yen and latex converts it to \textyen.

What happens then is independent of the input encoding, and equivalent to if you have \textyen in your source file. If that command is defined to produce a character it will (You could use textcomp package for example), but as the document font encoding is specified as T1 which does not have a yen, then by default you get the error that you show.

It is easier perhaps to see the distinction if you use [utf8] input, that allows the interpretation of thousands of input characters, but still if you are using T1 encoding the font only has 256 characters in it.