Inventory Items that have more than one project


I'm putting together (at least trying!) a database where we can give all the parts we use on projects our own part number.  These will be used to pull a bill of material for a particular item:  For example - Job A uses 4 parts.  Job B uses two of those same 4 parts.  Job C uses just one of those parts.  Job D has none.  

My question:  How do I add an inventory item and have it accessible to whichever jobs utilize it?  Some parts are used only on one specific job, and other parts are used on many different jobs.  I'm not sure where the one to many would be?