If your cat is shredding the arm of the sofa, it isn't being naughty. Scratching is normal cat behaviour, and a cat that can't do it somewhere sensible will pick somewhere for itself. The kind fix is not to stop the scratching. It is to give it a better home.
Scratching does several jobs at once. It helps a cat shed the worn outer layer of its claws, it stretches the back and shoulders, and it leaves a visual mark and a scent mark (cats have scent glands in their paws) that says "I live here". Cats also tend to scratch when they are excited, stressed or have just woken up. A move, a new pet or a change in routine can make it happen more.
That is why telling a cat off does not work. It does not remove the need, it just makes your cat wary of you. Vets and welfare groups agree on the alternative: offer something better to scratch, put it in the right place, and reward your cat for using it.
Cats are fussy about how they scratch. Some stand tall and rake downward, some stretch out flat on the floor, some like a slanted surface. The best clue is what your cat is already doing. Use the planner below to turn that into a shopping list and a placement plan.
Put the scratcher where your cat already scratches, make it tall and steady enough to stretch on, and reward every use.
Give it a few weeks and change one thing at a time: the spot, the height, the texture. Add a second scratcher in another room, since cats like several places, especially near sleeping spots and entrances. If your cat suddenly scratches far more than usual, it may be stressed by a change at home, so look for what changed and keep routines calm. If you notice sore, overgrown or broken claws, or your cat seems in pain, ask your vet.
Some owners ask about trimming claws. It can help, but ask your vet or vet nurse to show you how, and never cut near the pink part. Older or less active cats often need their claws checked more, because they wear them down less.
Declawing is not a nail trim. It removes part of the bone at the end of each toe, and animal-welfare groups and vets oppose it. It is banned or restricted in many countries, including most of Europe. If scratching is a problem you can't solve, ask your vet or a qualified cat behaviourist for help instead.
Sources agree on the basics; exact post heights are a rule of thumb, not an official standard.