Outlining is used during rug hooking to make a certain object(s) stand out when a viewer first glances at a finished project. Outlining is a technique that you'll see very often when having a peek around Deanne's studio, and her rugs contain varying amounts of the approach used in many different ways. The wider the cut of cloth you use and the more solid the colour, the more attention you will bring to that form once your rug is completed. Deciding when and when not to outline is a personal choice and only you can decide what you want to stand out in your creation!
The width of your outline materials greatly depends on what you're outlining, if it's a larger object then the width of your wool will be a little bigger, but generally speaking, smaller details aren't usually outlined.
The colour you choose for your outline should depend on the overall rug and should work within it, but still be distinct enough that it serves it's purpose. Outline colours can provide balance in a rug that might have lacked it before. Also, keep in mind that you don't need to use the same outline colour for every single object in your rug.
If you want your outline to really protrude....
-Use solid colours.
-Hook your outline very tightly, not skipping holes in your backing, this is really useful if you're outlining lettering.
-Hook your outline wool higher than the rest of your rug.
If you want your outline to be indistinct...
-Use yarns to hook your outline.
-Use materials with varying colours or patterns.
-Hook your outline loosely, skipping every few holes in your backing.
-Hook your outline lower than the rest of your loops in your rug.