See if you can tell what word should be changed in this product review:
(HP dv6-6013) Weighing 5.6 pounds, it’s a tad heavier than both the 5.3-pound Sony VAIO VPC-EB33FM/BJ ($629.99 list, 4.5 stars) and the five-pound Editors’ Choice Acer Aspire AS5742-6475 ($599.99 list, 4 stars).
The word “both” should be “either.” “Both” combines the two other items, “either” keeps them separate, and you can tell from the context that that’s what the reviewer means.
Here’s an example to help make this distinction easier to see: Suppose you could mop the floor with back-and-forth strokes of the mop, or sideways sweeps of the mop. You can mop the floor both ways or you can mop it either way. Mopping the whole floor both ways is twice as much work!