A correspondent writes to ask about the difference between tallest and highest in such sentences as Everest and K2 are the two tallest mountains in the world, which he has found on a BBC site. He quotes Michael Swan as an example of a grammarian who says that it has to be Mont Blanc is the highest mountain in Europe and not . . . the tallest mountain, and is understandably confused.
Usage is undoubtedly blurred, because we frequently see such listings (very common online) as 'The Tallest Buildings in the World', 'The Tallest Mountains in the World', and so on, alongside 'The Highest...' Tallest is four times more common than highest for buildings on Google. Highest is over ten times more common than tallest for mountains, but tallest still attracts a healthy 60,000 hits.
When we look for the reason, we enter a world of technical definition. Mountains are actually measured in three different ways. (1) Sea level to the peak. (2) Base to the peak. (3) Distance from the centre of the earth to the peak. The differences are significant. Under (1), the title goes to Mt Everest. Under (2) the title goes to Mauna Kea, Hawaii, which has only 4,245 m of its 30,000 m above sea level. Under (3) the title goes to Chimborazo volcano in the Andes, only 6310 m high but (thanks to the equatorial bulge caused by the earth's spin) further away from the centre than anything else. In this approach, (1) is referred to technically as the highest mountain, and (2) as the tallest mountain.
Most people would never need to refer to (3), and few to (2). We normally think of mountains as being above sea level, so this motivates the use of highest. But the fact that tallest is also a permitted collocation will have reinforced its appearance in everyday usage, resulting in the overlap noted by my correspondent.
Other domains raise similar issues. People normally talk about tallest buildings, referring to the dimension of the building from base to roof. The highest building in the world could be a single-storey house on some mountain-top somewhere. But again there is an overlap, and there are complications, well known in the field of encylopedia listings. Does the radio mast on top of a high building count as part of the height of the building or not? It can get quite controversial when competing claims are being made for the world record. And the complications reverberate linguistically when we encounter such usages as The highest part of the tallest building is well above roof level.