Plain has many meanings that are relevant to the falling of the angels. Plain means to mourn over something, which clearly refers to the angels saddened by their fate. Plain also refers to a vast open stretch of land which portrays Hell as huge and vast in area. Hell is huge and vast in order to fit the innumerable fallen angels but also implies that someone else will be falling from God's grace. The vastness of hell foreshadows that man will fall, sin and some will be damned to hell.