A modem designed to fit onto the handset of a rotary or touch tone (if you were lucky then...) telephone. In the day and age of acoustic modems, baud rates were not speedy and connections would be plagued by line noise. &���^

Connecting to the bbs you wanted to call into required you to dial in manually, listen for a carrier and then press a button on the modem. Then, the handset would be pushed into the modem's 'cups' and you'd be reading text feeds at a slower than molasses rate.

aka "acoustic coupler modem".

If you get one of these, make sure to pay the extra cash and buy a 300 baud one. 150 baud is just too slow!