What does Rb mean in coin rating?

When grading copper cents, there are three main subgrades: BN (brown, a tarnished cent) RB (red-brown, a cent that has began to tarnish) and RD (red, an untarnished cent) out of all of these, collectors will sometimes pay a premium for RD or RB cents because they are much more attractive. However, keep in mind that most slabs aren't airtight and your RB or RD cent may very well eventually tarnish into a BN cent, so my advice is not to pay a premium for the colour designation but rather focus on the true grade of the coin (expressed in a number from 1 being barely recognizable to 70 being perfection)