A Straightline Chuck is used to hold objects vertically, and allow for ornamenting them that way. The object being held:
moves up and down (along the Y axis),
rocks back and forth (the X axis), and
does not rotate around the lathe's spindle drive axis (the Z axis).
This is particularly useful when making ornamentation on cylinders like pens, or on long, flat objects like bracelets. Lindow Machine Works makes a very nice one that has even been used on an MDF rose engine lathe.
Just getting started? You can delay getting one of these.
Rose engine homemade, tour a guillocher
This video from Frédéric Verschoore shows a straightline chuck in use. This is around 13:00 minutes into the video, so it is set to jump ahead to that.
Ornamental Turning with the straightline chuck part 1
Ornamental Turning with the straightline chuck part 2
These videos are from David Lindow showing the straightline chuck on the Lindow-White rose engine lathe.
Disclaimer : eMail comments to me at OTBookOfKnowledge @ Gmail.com. The process of woodturning involves the use of tools, machinery and materials which could cause injury or be a health hazard unless proper precautions are taken, including the wearing of appropriate protective equipment.