1. Series: The Federal Reserve issued two different series of $1 bills in 1979 - Series 1977 and Series 1979. Series 1979 notes have a larger portrait of George Washington and a different Treasury seal compared to Series 1977 bills.
2. Print Run: The number of bills printed in a particular series can affect their rarity. Some print runs may be more limited than others, making certain bills more valuable to collectors.
3. Condition: The overall condition of the bill is crucial. Uncirculated bills in mint condition are generally worth more than circulated bills that show signs of wear and tear.
4. Star Notes: Bills with a star (*) symbol after the serial number are known as "star notes" and are generally considered more valuable to collectors due to their relative scarcity.
5. Error Notes: Any printing errors, misalignments, or other unique characteristics can make a bill more valuable to certain collectors who seek out such rarities.
6. Historical Significance: Significant events or anniversaries related to the year 1979 could add historical context to the bill and increase its value if it is associated with any notable occurrences.
7. Signature Combination: The combination of signatures on the bill by the Secretary of the Treasury and the Treasurer of the United States can be a point of interest for collectors, especially if it features uncommon or specific signatures.
It's important to note that the actual value of a 1979 US $1 bill can vary widely depending on specific details, such as any exceptional characteristics or unique traits. To obtain an accurate appraisal of the value of a specific 1979 $1 bill, it is recommended to consult with professional numismatists, experts, or reputable coin dealers who specialize in paper money.