The two "unusable" gears.
Back in the early '70s, when "ten speeds" were ubiquitous, the lore was that using the highest or lowest gears would cause the chain to fall off/get goofed up, therefore those gears weren't practical to use. (I don't remember the precise details of this bit of urban legend.)
Was that ever true?
I have a derailleur seven-speed, but the gear changer is a clicker set up on the handlebar, not levers like they had in the old days.
~~~A bicycle without fenders is a person without eyebrows.