[SQL] underscore problem

[postgresql]

Is there anywhere in the docs that says we should not use an 
underscore in a table name?

table  FC_Client_Info exists with a record where jobno 1234 has 
info:

select acode from FC_Client_Info where jobno = '1234';
fails with a warning that fc_client_info does not exist
notice the lower case. If I quote the table

select acode from "FC_Client_Info" where jobno = '1234';
the select works fine

What's the problem here? This should be easy.

Ted