Is it grammatically correct to say:
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This book is twice as expensive as that one.
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That book is twice as cheap as this one.
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This rope is twice as long as that one.
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That rope is twice as short as this one.
I was studying comparatives and on some forum I found a discussion about expressions like "twice as few" and "twice as little". On that forum native English speakers all have agreed that such expressions sounded weird to them, and made no sense, because "twice" means multiplication and is opposite in meaning to being smaller. For example, it is incorrect to say: "I have twice as few books as he." And so I was wondering if "twice" is also opposite to being shorter, cheaper and slower, etc. Could anybody please elaborate on how "twice as few" is incorrect while "twice as short" is correct?
Best Answer
I agree with Sam K's answer — the sentences using "twice as cheap" and "twice as short" are correct, but unusual compared to the sentences using "twice as expensive" and "twice as long". I also agree pretty much with the reasons in that answer, that the two sentences have different implications because of something that's different about the adjectives. I want to clarify a little bit in the details, though.
When you say "X is twice as Y as Z" (for example "Texas is twice as big as Montana"), there's an implication that Z already has some quality of Y, but X has it twice as much. That is, Montana is big, but Texas is twice as big. With some adjectives (the ones Sam K calls "strong"), this implication is weakened because they're the ordinary adjectives used to describe the extent of something. If you want to know the size of something, you can ask "how big is it?" If you want to know the length, "how long is it?" If you want to know the cost, "how expensive is it?". So you can say "twice as big as an ant" without implying that an ant is very big at all. It's already understood that a thing has some size, some length, some cost.
But when it comes to adjectives like "small", "short", or "cheap" (what Sam K calls the "weak" adjectives), something can only be small, short, or cheap in comparison to something else. If there's no obvious target for comparison, then it's compared to something that would be considered normal or average in the context of the conversation. So, "these apples are twice as cheap as those ones" usually implies that those apples are cheaper than usual (for apples, or for fruits), but these ones are even cheaper. It's impossible to have the same kind of mathematical precision you do with "twice as expensive"; there's too much inferred and unstated.
If you want that effect, great! If you don't, then you might want to use "half as long" instead of "twice as short", "half as expensive" instead of "twice as cheap", etc. They're more neutral, more precise, and more common.