What is difference between hardboard and thin MDF?

I will mention that I am in the UK in case the terms are different in different countries.
What is the difference between hardboard and thin MDF (medium density fibreboard)?
I know hardboard as usally being dark brown, about one-eighth inch thich, shiny on one side, often criss-crossed on the rough side and rather too easy to sanp if you bend it too far.
I went to get some hardboard from Focus and saw there was hardboard and there was also some very similar-looking thin MDF board. In fact I thought they had mis-shelved some stock and had put hardboard in the MDF slot but the label on the sheet said MDF.
The MDF cost rather more than the hardboard.
What is the difference?
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Start from similar beginnings of scrap, but different manufacturing process. MDF has more resins.
http://www.design-technology.org/hardboard.htm
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Not much. They're both fibreboards of similar density.
Hardboard is made by an older process. It's one sided and the density varies so that the smooth side is more impact resistant than MDF and the rough side is less impact resistant. If wetted, hardboard warps, MDF expands and delaminates.
You can often interchange them.
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In an earlier contribution to this discussion,

Hardboard has a hard smooth side and a soft corrugated side, and is not very strong overall. MDF is much more homogeneous, stronger, less compressible, and looks the same both sides.
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You have to define stronger. There are differences in flexural, compressive and shear strength. IMO, hardboard has better flex qualities.
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In an earlier contribution to this discussion,

Agreed. I was thinking more of tensile and shear strength. It's easy to fold and break hardboard over a hard edge - MDF puts up more of a fight.
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And which is best to use as an underlay for vinyl/laminate? I've always thought hardboard ...
a
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In an earlier contribution to this discussion,

up irregularities in the sub-floor, whilst giving a pretty flat top surface. MDF will bridge over irregularities, and feel springy when you walk on it.
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