Termology: Drop Letter

I just got back from my stamp club meeting. We watched an APS slide presentation about a "King Is Coal" topical collection. In the presentation the term "DROP LETTER" was used frequently in regards to late 18th & early 19th century mail and I am not sure why and what it exactly meant? I assume it means that a letter collection box was used and not a letter taken directly to a postal clerk for cancellation.

Any idea?

A letter that is mailed and delivered from the same post office. (The free dictionary by Farlex)

Search text string = Drop (from askphil)

Drop: USPS term for lobby slot or opening where customers deposit mail.

Drop addresses: scheme used to circumvent restrictions on mail between countries at war whereby
mail from one country to another was routed through a neutral country; address was crossed out and
resent to the true destination; also known as undercover addresses.

Drop letter: letter delivered from the same post office where originally posted.

Dropped letter: 1: a missing letter in typesetting caused by letter being lost of broken off during
printing. 2: a letter which is out of alignment with rest of letters; usually found in typeset overprints.

Dropped mail: mail dropped from the air for forwarding to destination.

Dropped transfer: an impression on an engraved printing plate occurs when a transfer roll touches
the blank plate before being positioned correctly.

You often see a cover addressed to John Smith, 23 First St., City.

That's usually a drop letter.

PS There was a time (in Canada and likely elsewhere) when a drop letter had a low rate of postage.

That's really neat, thanks Rod! I never knew that.

It just goes to show that knowing why there were different stamps in a set of stamps (to pay the different delivery rates) allows one to appreciate their collection that much more.

Very interesting! Thanks everyone!

You were supposed to raise your hand

Teachers,
Kirk

That's right Puzz,
specialist collectors in Aust are making big dollars here
identifying correct usage covers.

another reason never to soak stamps off covers.

Drop letter? Not sure of the postal rate, but I know the story:

A-tisket a-tasket
A green and yellow basket
I wrote a letter to my love
And on the way I dropped it,
I dropped it,
I dropped it,
And on the way I dropped it.
A little doggie picked it up and put it in my pocket.
Then I bought a stamp and posted it.

I have no idea why I found that amusing,
but I cackled for several minutes :)

Wow! I dropped by youtube and watched Ella Fitzgeralds's
rendition.
Aaaah super stuff!

was it Blue!? No.No.No........

OK, I added that line

Kirk,
Even if I raised my hand, the slide program could not answer my question. LOL. Actually, several others present were not sure of the definition of the phrase either.