system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do banks know that it’s not you using your card when they send out those fraud alert messages?

So I just woke up this morning, made my coffee and was scrolling through Reddit.. And I got a Fraud Alert text from Chase telling me they declined a purchase of $153.60. I buy stuff *all* of the time with my chase debit card.. online, in store, sometimes run it as credit.. and I never get those fraud alert messages. 


But, the first time that someone somehow stole my card numbers and tried to make a purchase, they declined it and flagged it as fraud. And thank god they did.. but how did they know it wasn’t me?
gpt
The credit card company knows a good deal about you.  They know where you live, where you usually shop, what you usually buy, etc.  If they detect a purchase that deviates from your patterns, they will flag it as potential fraud.

So, if you normally buy food, gas, groceries, etc. on your card in one particular part of one city, then someone buying electronics in a different city is going to raise flags.