On any bike with derailleur gears (bikes with multiple sprockets on the front and/or back); there are two gears that you should ride in rarely if at all. These are called crossover gears because when the bike is in these gears, the chain crosses over from the extreme left or right on the front to the extreme right or left on the rear. In these positions, the chain is most likely to wear the cogs in back and chain rings in front and it’s most likely to make noise and miss-shift. So, pay attention and try not to shift into the large chain ring/large cog and the small chain ring/small cog combinations.