Depends on where you lived.

In some places, you’d leave a note with someone you knew the recipient was going to see that day. In some places, note passing was quite an artform, including special paper folding to protect the contents from prying eyes.

In Paris, they had the tubes — the entire city was plumbed with vacuum tubes, and you could write a note or even pack a small object into a capsule with an address, drop it in your local receptacle, and it would zip across town in minutes to the recipient address.

In other places, markings on trees, mirrors, flags, smoke signals and various musical instruments and bells have been used.