Simple answer: No tuning of any channel without a box of some kind - Set top tuner for antenna, Cable box for cable, or Satellite box for satellite.
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The tuner in a HDTV set with the built in tuner means that the set works with an antenna (aerial). Some tuners in US HDTV sets allow access to additional unscrambled cable channels or cable channels with cable card (as authorized by the cable company).
No tuner HDTV sets were built so that manufacturers could sell what is a High Definition monitor for people with Cable Boxes, Satellite Boxes, Set Top Boxes (the missing tuner) and DVD players with High Definition outputs via the HDMI connection or a Component Video and Stereo Audio Connection.
There was a reason for manufacturers to sell the no tuner HDTV sets, the cost of the HD tuner was high due to the royalty that had to be paid to the people who developed the tuner (at least the ATSC tuner in the US). And the sets had to have both the analog (NTSC) and digital (ATSC) tuners if the set had either one up to a certain size (until March 1, 2007 when all US sets made had to have both).
The only sets in the US which can legally still be sold without a ATSC (digital SDTV or HDTV) tuner must have been made before March 1, 2007 (and be below a certain size since larger sized sets were phased into this requirement earlier)
Must have no tuner at all and be called a monitor.