Gdtyjr

Sometimes the margins need to be changed for a particular page, paragraph, or other section of text. For example, if I'm writing a letter and want the left margin to be almost at the right side of the page for the four lines of my address, or if I am combining prose with poetry and want the poems to be indented relative to the rest of the work.

What is the best way to change both left and right margins on the fly?

There are several packages available on CTAN to do this. changepage looks promising but you can find other alternatives by searching for "margins" or "changepage" on ctan search.

With the changepage package, you can use the adjustwidth environment as follows:

beginadjustwidthleft amountright amount
lipsum[2]
endadjustwidth

For example, to remove 100pt from the margin on both sides, you would use

beginadjustwidth100pt100pt

Here is how you can do it. Put the following in the preamble (before begindocument)

defchangemargin#1#2listrightmargin#2leftmargin#1item[]
letendchangemargin=endlist 

then in the text you can use

beginchangemargin<arg><arg> 
endchangemargin 

where <arg> is the distance you want to include on the margin (the first one defines the right-hand side margin, and the second defines the left-hand side one).

So, for example, to add 0.5 cm to the margins on either side, you would have:

beginchangemargin0.5cm0.5cm 
%your text here 
endchangemargin

This is exactly how the command

beginquote
endquote

is defined, but with the set to 1cm. The command quote can be used without having to load any packages, by the way.

With either one of the KOMA-Script classes or the package scrextend (which is part of KOMA-Script), you can use the addmargin environment.

documentclassarticle

usepackagescrextend

usepackage[english]babel
usepackageblindtext

begindocument

blindtext

% Syntax: beginaddmargin[<left indentation>]<indentation>
beginaddmargin[4em]1em
blindtext
endaddmargin

blindtext

enddocument

what about the narrower (TeX?) command? Is it ok to use even though it does not offer a very precise control?

documentclass[11pt]book
usepackage[latin1]inputenc
usepackage[a4paper,top=3.5cm,bottom=3cm,left=3.6cm,right=3.6cm]geometry
usepackagelipsum
begindocument
lipsum[4]
narrowerlipsum[4]
par
lipsum[4]
lipsum[4]
lipsum[4]
lipsum[4]
narrowernarrowernarrower lipsum[4]
par
lipsum[4]
enddocument

It might not answer the question directly, but:

There is a letter class:

documentclassletter

for writing letters.

And for verse there is a package called...wait for it...verse:

usepackageverse

As I said, it doesn't answer the specific question, but it might solve the two applications you want the solution for.