On Thu, 14 Apr 2005 09:48:07 GMT, news wrote:

Hi,

I always thought it was to limit the chances of the terracotta cracking when
the soil inside freezes during inclement weather. The frozen soil is forced
up and therefore does not exert to much pressure on the off vertical sides.

Sure, any sloping-side will do for that.
The OP was asking about the thickened ring around the top lip, not what are
pots tapered. As I understood it.

--
Tim C.