I can run the following simple query on the standard Account and Contacts object to include values from the parent object when querying the child object.

select firstName, lastName, Account.Name
  from Contact
  where firstName like 'Vinn%'

However, when I try to do this on a custom object I created, Custom_Account__c, I can't get it to work at all.  I set up the lookup field on Contact to have the same relationship as it does with the regular Account object (parent/child).

I tried the following query and many other permutations including putting the relationship name (Contacts) in place of Custom_Account__c, doing this with and without "__r" appended to it.

select firstName, lastName, Custom_Account__c.Number_Field__c
  from Contact
  where firstName like 'Vinn%'

I tried all sorts of variations but I keep getting the error:

"Didn't understand relationship 'Custom_Account__c' in field path. If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name. Please reference your WSDL or the describe call for the appropriate names."

I don't understand why I can't simply reference the fields in a custom parent object as I do with a standard parent object.

Thanks!