 Historically, amateur stations have rarely been allowed to operate on frequencies lower than the medium-wave broadcast band, but in recent times, as the historic users of these low frequencies have been vacating the spectrum, limited space has opened up to allow for new amateur radio allocations and special experimental operations.Since parts of the 500 kHz band are no longer used for regular maritime communications, some countries permit amateur radio radiotelegraph operations in that band.Many countries, however, continue to restrict these frequencies which were historically reserved for maritime and aviation distress calls.