What frequency should you tune into when instructed by the tower controller to contact ground on "point niner"?

When instructed by the tower controller to contact ground on "point niner," you would interpret this as the frequency ending in .9, which corresponds to 121.9. This is a standard frequency used for ground communications at many airports. The number "niner" represents the phonetic way of stating the numeral 9 in radio communications, adhering to the conventions used to avoid confusion over similar sounding numbers.

The context of using "point niner" emphasizes the importance of clarity and precision in communications, which is crucial in aviation operations. Frequencies like 119.9 or 120.9 are also standard frequencies but serve different purposes, such as air traffic control or local communications, whereas 122.9 is typically used for common traffic advisory frequencies at uncontrolled airports. Thus, tuning into 121.9 ensures that the pilot is able to communicate effectively with ground control as per the tower controller's instructions.