Setting margins using geometry package

Question

How can I change the four margins differently using the geometry package ?

Currently I have got

{\geometry{
   paperwidth=21cm, paperheight=21cm
   textwidth=20cm,  textheight=20cm}}

Answer 1

The instruction

\geometry{ paperwidth=21cm, paperheight=21cm,
           textwidth=20cm,  textheight=20cm}

can also be expressed as

\geometry{ paperwidth=21cm, paperheight=21cm, margin=5mm }

To set asymmetric vertical and/or horizontal margins, you could run, say,

\geometry{ paperwidth=21cm, paperheight=21cm, 
           left=3mm, right=7mm, top=4mm, bottom=6mm }