Jacquard or Fair Isle Knitting is a technique that allows for working with 2 colours at the same time, but such that only one colour is used at a time, while the other colour runs or floats on the wrong side of the fabric.
When knitting a Jacquard pattern, it is important to learn how to hold the yarns in the right way across fingers. One yarn is held across the index finger and the other yarn is held across both index and middle finger.
This offers a number of advantages:

1: The yarns are never twisted while knitting, as they have no connection.
2: The yarns run evenly together and the floating yarn gets exactly the right length.
3: The yarn that lies across the index finger, i.e. to the fore of the knitting direction, will automatically stand out in the pattern. It will dominate the other colour and hence we call it the .

Jacquard knitting is easiest to knit on a circular needle. With a little practise, it becomes possible to work a WS row. It is important, however, to change the yarns around, so the yarn dominant on the RS rows change place with the other yarn used and the latter then becomes the dominant on a WS row.