Sentence:  It is usual, customary and correct for the <WSD>house flag</WSD> of a company to be worn at the mainmast head; however there are exceptions, and as Commander Hilary Mead has pointed out in his book on Sea Flags, the Brocklebank Line have always used the foremast. 
Meaning of the word marked with <WSD> tags:  Flag that identifies a shipowner.