I am trying to create a query to group our Project Billing Item (custom object) by Project (another custom object).  Below is a copy of my initial query:

SELECT Name, (SELECT Project__c, Name, Employee__c, Description_of_Work__c, Hours_Worked__c, Date__c FROM Project_Billing_Item__c) FROM Project__c

Which generates the following error when executing: "Didn't understand relationship 'Project_Item__c' in FROM part of query call.  If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name."

I also tried changing it to the following:

SELECT Name, (SELECT Project__c, Name, Employee__c, Description_of_Work__c, Hours_Worked__c, Date__c FROM Project_Billing_Item__r) FROM Project__c

....and still receiving same error message stated above.  If it helps Project Billing Item (Project_Billing_Item__c) has a Master-Detail relationship to Project (Project__c).  Some additional information in case it helps:ks

Master-Detail Options
Related To:   Project
Child Relationship Name:   Project_Billing_Items
Related List Label:    Project Billing Items

Thanks for the help and if I can provide any additional information that might help just let me know!