The structural network of calfskin, goatskin, sheepskin, and pigskin is presented, the particular examples being relevant to the care of leather antiquities. The differences between grain and flesh sides are described, and the problems with applying dressing to these surfaces become obvious. Leather used in repair must also be selected with reference to the leather fiber characterisics. Adhesive in contact with leather results in damage and an undesirable appearance. See also AATA 15-912. -- AATA