The lines the card must carry
The card exists for one scenario: you are unconscious, hospitalised or otherwise unable to speak, and a responder, paramedic or hospital staff member opens your wallet looking for who to call. Its first line must therefore do the whole job at a glance: a clear statement such as emergency, I have a cat at home alone. Under it, your full name, so the card can be matched to you, and then the cat's essentials: name, breed or type, and a one-line physical description plain enough for a stranger to confirm they have found the right animal.
The most important entries are the humans. List two emergency contacts, with mobile numbers, who have agreed in advance to be on the card, and note which of them holds a key or another way into your home. One contact alone is a single point of failure on the day they are on holiday. Add your home address, since a contact who is feeding the cat needs it in writing if they usually visit by route rather than by address, and your vet's practice name and number, so whoever steps in can reach professional help and the cat's records in one call.
The care details worth one line each
Under the contacts, add the minimum care summary that keeps the cat safe for the first days: how often the cat is fed and roughly what, where food and litter are kept in the house, and anything time-critical phrased in general terms, such as a note that the cat is under vet care and the practice should be rung for instructions. If you have more than one animal, say so plainly with a count and names, because a helper who finds one cat in a two-cat house will stop looking too early.
Mention where the cat hides when frightened. It is the detail nobody thinks to write down and the one a stranger in your home needs most, since a cat missing from view may be assumed to have escaped when it is actually behind the boiler. Note also whether the cat is indoor-only, so a helper knows to guard doors rather than let the cat out as usual.
What to leave off, and making the card work
Leave off anything sensitive or useless in a crisis: no bank details, no alarm codes written in full, no long medical history of either you or the cat. The card is a pointer, not a file; its job is to connect the cat to the right people fast. Keep the writing large enough to read without glasses in poor light, and prefer a printed or boldly hand-lettered card over small neat script.
Tell your named contacts the card exists, and put a matching note on the fridge at home so anyone who reaches the house finds the same information in the same words. Replace the card whenever a contact changes number, a vet changes, or the cat's care changes materially; an out-of-date card is worse than none because it sends help down dead ends. Your cat's side of the system matters too, and this is where the card and the cat's own identification join up: responders who find the card can be directed to the cat's KinsLe Pet Pass, its verified ID card holding the cat's identity and your contact details ready to show anywhere.