#sill  structural member consisting of a continuous horizontal timber forming the lowest member of a framework or supporting structure
  supertype:  #structural_member  support that is a constituent part of any structure or building
  subtype:  #doorsill__doorstep__threshold  the sill of a door; a horizontal piece of wood or stone that forms the bottom of a doorway and offer support when passing through a doorway
  subtype:  #windowsill  the sill of a window; the horizontal member at the bottom of the window frame

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