What is pooling yarn?

Pooling is something that yarn does, not something yarn is. It is usually undesirable, but sometimes planned.

Multicoloured yarn in red, blue, white, yellow, black, purple and pink.
Multicoloured yarn

If you have a yarn like this, the colours can form unwanted blobs, instead of spreading out in a nice way:

Brown and white yarn, pooling into blob of colour.
Yarn pooling

This is a massive problem with variegated or multi-coloured yarns, especially those gorgeous handpainted skeins you picked up at a wool festival for a small fortune, only to realise they look prettier in the skein than they ever will knitted up (~looks mournfully at the “special” shelf in her craft room~).

Some of my special variegated yarns, lying their in their skeins, never to be knitted - only petted and drooled over.
Whyyyyyyyyyy???????????

BUT: if the colour changes are regular, you can do a thing called ‘planned pooling’, which produces effects like this:

3 images of planned pooling, revealing complex multicoloured plaids, argyles and checks. All were worked in variegated yarns with regular colour changes.
Planned pooling

instead of this:

A sweater with unplanned pooling, where the yarn has broken into swirls and patches of colour.
Unplanned pooling.

Here’s how:

Quora linky.