Originally Posted by demoncyclist
You want to stay away from the cross-chain combos: Big ring/big cog and small ring/small cog. They can place too much side load on the chain, and can cause rapid drivetrain wear.
I'll add that this combination you don't want to stay in... Sometimes when trying to shift around or so and you may end up in one of these combinations is OK... Just know you don't want to be there and take slow but immediate action to remove yourself from that condition.