Blood-type abbreviations are commonly used to identify a person's blood type. The most common abbreviations are POS for positive and NEG for negative. These abbreviations are used to indicate whether a person has Rh antigens in their blood, which can be either positive or negative. People with Rh-positive blood have the antigen present, while those with Rh-negative blood do not. Knowing a person's blood type is important for medical purposes, such as when receiving a blood transfusion.