I only brush my DLH once a week or so, and she very rarely gets knots. Usually after she's been climbing in the confiers at the back of our house chasing birds

A greyhound comb is brilliant for getting down to the roots to remove the tangles before knots are allowed to develop. A brush can then be used to finish off.

In Summer a long haired cat loses the undercoat which causes the coat to knot if not combed or brushed out properly. More attention to combing/brushing is needed when the coat is coming back in early Spring and Autumn, especially under the armpits, belly and inbetween the legs

Another useful grooming tool for longhairs is a soft slicker brush which will also pull out the small knots from the undercoat.

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That's really useful to know

Does anybody have any tips for getting matts out? Ace has one under his armpit at the moment and refuses to sit still for me to get to it... I'd rather not have to go down the cutting out route.

Try a seam ripper, its a useful little gadget for breaking up knots. You can get one of those from a haberdashery.
It might be better trying to pick the knots apart gradually with your fingers as the underarm areas are very sensitive and no doubt your cat will put up a lot of resistance to you doing anything there.
Also if you try and keep the coat as grease free as possible by sprinkling some baby talc under the arms and any other areas that are prone to knotting, then brushing it out well.

Never attempt to cut a knot that is close to the skin using scissors.

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Thanks Angeli

I'd have taken him to see the vet nurse before cutting it out tbh. I'm a wuss about hurting him, I'm terrible when it comes to nail clipping time