Watermarks:
One of the primary security features of U.S. bills is the watermark. This is an image embedded into the paper itself and can be seen when held up to the light. Modern U.S. bills, especially those of higher denominations ($10, $20, $50, and $100), feature watermarks that match the portrait on the bill. For example, the $100 bill has a watermark of Benjamin Franklin. Watermarks are difficult to replicate accurately, providing a reliable method to check the authenticity of a bill.

Color-Shifting Ink:
Another key security feature is the use of color-shifting ink. This ink is used in the numeral at the lower right corner on the front of the bill. When you tilt the bill back and forth, the color shifts from copper to green. This feature is present on denominations of $10 and higher and helps prevent counterfeiting by adding a layer of complexity that is hard to reproduce with standard printing technology.

Microprinting:
Microprinting involves printing tiny, hard-to-reproduce text on various locations of the bill. On genuine U.S. currency, microprinting can be found around the portrait, in the security thread, and on the borders. For example, on the $100 bill, phrases like "USA 100" can be found printed in tiny letters. These microprints are extremely difficult to replicate accurately with standard printing techniques, adding another hurdle for counterfeiters.

Security Threads:
Each U.S. bill of $5 and higher contains a security thread embedded in the paper. This thread runs vertically and is visible when held up to the light. The security thread is imprinted with the denomination of the bill and includes a series of microprinted text. It also glows under ultraviolet (UV) light, with each denomination glowing a different color (e.g., $20 bills glow green, $50 bills glow yellow). This feature helps both the public and machines verify the authenticity of the bill.

3D Security Ribbon (on the $100 bill):
The $100 bill has an advanced security feature called the 3D security ribbon. This blue ribbon is woven into the paper and contains images of bells and 100s that shift as you tilt the bill. The bells change to 100s and back again, creating a visually striking effect that is difficult to duplicate.

Enhanced Portraits and Watermark Images:
The redesigned portraits on modern U.S. currency are printed with such high detail that they resist replication by standard copying and printing methods. The watermark images embedded into the bills also align with the primary portrait, making it harder for counterfeiters to match both elements accurately.

Color and Pattern Variations:
Recent redesigns of U.S. currency include subtle background colors and patterns that add aesthetic complexity and security. For example, the $20 bill has a subtle peach color in the background and intricate line patterns that make it more challenging to replicate without sophisticated printing equipment.

Infrared and Ultraviolet Features:
Some security features on U.S. bills are designed to be visible only under infrared (IR) or ultraviolet (UV) light. Special markings or threads on the bills can be detected by machines using IR or UV light, providing an additional layer of security that is not easily seen by the naked eye.

These secret security features, continuously updated and enhanced, help maintain the integrity of U.S. currency. They ensure that genuine bills can be easily verified while making counterfeiting increasingly difficult.