A lucet is a two pronged implement that makes some very nice braids with multiple uses. They were used from at least the 16th century onwards, however some viking finds of manmade bone objects shaped like lucets have been used as evidence of viking use of this technique. No one has been able to prove the useage of the lucet between these two time periods yet, however finds of other braiding techniques including plaiting and fingerloop braiding have been documented between the 10th and 16th centuries.
The lyre-shaped lucet is technically out of period (though prettier and easier to use). Instead, early lucets were generally a simple tube with prongs at the top ends (most finds being a hollowed, carved bone).
They can also make elliptical cross-section braid using the "twisted-stitch:" method that's pretty good for couching cord - as long as it's made with a fine fibre.
Braids can be made with multiple colours - either interchanging on individual stitches or groups of stitches, or used as a gimp thread for decorative purpose. Beads can also be added either in straight rows or looping around the braid in a helix pattern.
Braids made on a lucet need to be made from thread that can take a bit of abrasion, as the technique will cause the thread to rub against the horns of the lucet when the loop is pulled tight.
Instructions (including how to make 2 colour and beaded cords) are available at this page: http://www.stringpage.com/old/lucet.html
Another excellent resource is the Lacis book: "Lucet Braiding: Variations on a Renaissance cord" by Elaine Fuller ISBN 18911656066
You can get a lucet from: http://merchants-medieval.com/cockerel/