Improvement in high-frequency (HF) communication and frequency management, especially at high latitudes, depends on knowledge of ground ranges accessible to propagating modes and frequencies not only at the maximum usable frequency (MUF) but also at frequencies below the MUF, these lower frequencies being often used in communication to assure operation below about 90% of the population of observed MUF values. To this end, calculations from an advanced HF ray-tracing program, “AMBCOM,” of receivable frequencies for basic HF ionospheric modes at the high latitude station Andoya, Norway are plotted in the frequency/ground-range plane, for a chosen set of geotemporal conditions and sunspot number. The plots are made for transmitters located on northern, eastern, southern, and western bearings relative to Andoya, and for ground ranges of up to 6000 km from this assumed receive site. This method of representation of HF ray trace calculations results in the delineation of the regions of occurrence in the frequency/ground-range plane of the main HF propagation modes for chosen conditions. Such plots are appropriately termed mode structure diagrams. The HF mode structure diagrams provide valuable information, discussed herein, pertaining to (1) the determination of what modes and frequencies are best suited for specific paths and ground ranges at high latitudes, (2) identification of factors affecting the performance of HF communication at frequencies below the MUF, and (3) the conception and performance of further studies to resolve certain outstanding HF communication problems. Approximate formulae for the frequency range available in 1-hop F2 modes at high latitudes, motivated by mode structure diagrams, are developed and discussed.