The US Doubler was an enhancement for the Atari 1050 disk drive.

As supplied, the 1050 was a single sided, 40 track, "dual density" disk drive (although everybody cut extra notches in their disks to use it as a "flippy disk").

In single density mode, it used 128 byte sectors and had a capacity of about 87 K / side. Dual density still used 128 byte sectors, but about 50% more of them, giving a capacity of about 127 K. For a machine with only 64 K of RAM, this was a reasonable amount. Having a serial connection at 19.2 Kbps, data transfer wasn't very fast - about 1.8 K / second.

The US Doubler was two chips that were a user upgrade as long as you were reasonably proficient with a screwdriver. It came with very detailed instructions - basically open the drive up, unscrew a faraday cage, remove two chips, insert the two new ones, and put it back together.

You then boot up with Atari DOS 2.5 and... nothing is different!

However, boot up with SpartaDOS and you get true double density (same number of sectors as single density, but 256 bytes each - capacity of about 180 K). You also got high speed serial transfer, increasing the throughput to about 3 K / second.

SpartaDOS and the US Doubler were often sold as a package to "serious" Atari 8 bit users.