Horizontal scan rate, or horizontal frequency, usually expressed in kilohertz, is the frequency at which a CRT moves the electron beam from the left side of the display to the right and back, and therefore describes the number of horizontal lines displayed per second. CRT timings actually include some horizontal scans before the visible display, after it, and during the travel from bottom to top (known as vertical back porch, vertical front porch, and vertical sync width, respectively, and collectively known as vertical blank time), so the horizontal scan rate does not directly correlate to visible display lines, unless the unseen lines are also known, but it can still be used to approximate the display lines, as the total blank time is usually a small but significant portion of the total lines.