The Accession number identifies the donation of a group of objects from the same person at the same time; the catalog or object ID number is the number assigned to each object within that collection. The catalog number is usually in three parts, separated by a period. Example: 1986.4.3 ; the first group of numbers are the year it was donated,the second identifies the fourth group of 1986, and the last number identifies the object within that donation. The first two number groups typically make up the Accession number (4th donation of 1986). No matter what you numbering system, the Accession number points to the group; the three identify the object.