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 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.
It is usually the most limiting factor of a CRT display. This limit is due to how quickly the electromagnetic deflection system can reverse the current flowing in the horizontal deflection coil in order to move the electron beam from one side of the display to the other. Reversing the current more quickly requires higher voltages, which require more expensive electrical components. The time allowed by the standard was chosen to allow practical TV receiver designs.
Compare vertical scan rate (refresh rate), which indicates how often the electron beam is moved from the bottom of the display to the top. Given the horizontal scan rate, the refresh rate can be approximated by dividing the horizontal scan rate by the number of horizontal lines multiplied by 1.05 (since about 5% of the time it takes to scan the screen is spent moving the electron beam back to the top). For instance, a monitor with a horizontal scanning frequency of 96 kHz at a resolution of 1280×1024 has a refresh rate of 96,000/(1024×1.05) ≈ 89 Hz (rounded down).
In analog television systems the horizontal frequency is between 15.625 kHz and 15.750 kHz.