Intarsia is a knitting technique where you knit with blocks of colour. When you change from one colour to the next, you twist the two colours of yarn around each other so that the separate blocks of colour are held together.
When you knit intarsia, you don't carry the colours of yarn you are not using across the back of your work (like you do with fair isle knitting). Instead, you have a ball of yarn for each block of colour you are working. These individual balls of yarn are known as bobbins.
I used intarsia to knit the cloud in the photo. I used three bobbins of yarn: one blue bobbin for the left side of the cloud, one white bobbin for the cloud, and one blue bobbin for the right side of the cloud. I started by knitting the blue rows below the cloud with one blue bobbin. When I started knitting the cloud I joined in the white bobbin and then the second blue bobbin. For joining in new yarn have a look at my how-to guide for joining another yarn colour.
When you change from one yarn colour to the next, you twist the two yarns around each other once to link them together. You then carry on knitting with the new colour, and leave the old colour waiting until you come back to it on the next row.
Usually you work flat when knitting intarsia. It is unusual to knit intarsia in the round.
Intarsia patterns are often described in knitting charts. The image below shows the chart for knitting the cloud in the picture. When working from a chart, on a right-side row you knit from right to left across the chart, and on the wrong-side row you knit from left to right. Each small square on the chart represents a stitch. The chart shows what the finished knitted fabric will look like.