A crash/ride cymbal is a medium weight, slightly tapered cymbal, normally in the 18–22-inch (460–560 mm) range, designed to serve in a drum kit as both a crash and a ride cymbal.
A ride/crash cymbal may be thought to be the same thing, but is actually different. Despite being similar in design and function to a crash/ride, it is slightly heavier and/or less tapered to optimise the ride rather than the crash function. It is far less common than the crash/ride.
Crash/ride and ride/crash cymbals have several uses: