This article is from the RSC/UKSC Cricket FAQ, by Ganesh with numerous contributions by others.
When a batsman takes his usual side-on stance, the side
towards his legs is his strong side and is called the "on"
side or leg side of the field. The other side is called
the "off" side.
For a Right Handed Batsman(RHB)
-----
| | | stance of a right-hander
| | | /
offside<----| | | /
| ||| |/
| || | --->legside
--' --' or onside
-------
| | | stance of a left-hander
| | | /
| | |---/-------->offside
| | || | /
legside<-------| | /
or onside `-- `--
 
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