The forehead lines refer to the horizontal lines that develop above the eyebrows. These are present because of the repetitive action of the frontalis muscle situated above the eyebrows stretching up past the hairline.
These lines can, if pronounced, give a surprised appearance. One of the most common areas to be treated with botulinum toxin, it can take years off someones appearance.
Injections are placed in the frontalis muscle in two horizontal lines in order to inactivate this muscle and produce the best results.