The plural form sancta sanctorum is also used, arguably as a synecdoche, referring to the holy relics contained in the sanctuary. The Vulgate translation of the Bible uses sancta sanctorum for the Holy of Holies. Hence the derivative usage to denote the Sancta Sanctorum chapel in the complex of St. John Lateran, Rome.
The Latin word sanctum is the neuter form of the adjective "holy", and sanctorum its genitive plural. Thus the term sanctum sanctorum literally means "the holy of the holy ", replicating in Latin the Hebrew construction for the superlative, with the intended meaning "the most holy ".
Romance languages tend to use the form sancta sanctorum, treating it as masculine and singular. E.g., the Spanish dictionary of the Real Academia Española admits sanctasanctórum (without the space and with an accent) as a derivative Spanish noun denoting both the Holy of Holies in the Temple in Jerusalem, any secluded and mysterious place, and something that a person holds in the highest esteem.
Some regional branches of the Catholic Church, e. g. Germans, are wont to refer to the Blessed Sacrament, when adored in the tabernacle or in exposition or procession (e.g. on Corpus Christi), as the Holy of Holies. By custom, It is adored with genuflection; with a double genuflection, that is a short moment of kneeling on both knees, if in exposition; in the procession this ritual may be nonrigoristically alleviated, but at least a simple genuflection is appropriate when It is elevated by the priest for blessing or immediately after transsubstantiation. Personnel in uniform — which in Germany includes student corporations — give the military salute when passing by or in the moment of elevation.