From D'Addario:

As a half round phosphor bronze acoustic guitar set, Flat Tops are designed to significantly reduce unwanted finger noise. Combining tried-and-true phosphor bronze tone with a smoother feel and quieter playing experience, they’re excellent for recording—and slide guitar playing. Flat Tops are made with a high carbon steel core and phosphor bronze wrap wire, which is then precision ground and polished, leaving behind a semi-flat string with a smooth feel and beautifully balanced tone. 12-53 Light gauge strings are our most popular acoustic gauge, providing a nice balance of playability and tone.