metric v modular building block sizes?

Metric sized building blocks (commonly 440 x 100 x 215mm) have been around since metric sizes arrived in the 70s.
What is the application for 'modular sizes' (eg 390 x 100 x 190 - even 390 x 90 x 190)?
eg
http://www.plasmor.co.uk/html/products/plasmor-product-selector/concrete-blocks/lightweight-aggregate-blocks/aglite/?PHPSESSID f648add4e125646c033e7d91fcfcf4
Is a new standard being introduced? Most block manufacturers websites show this size in addition to the metric ones.
Also, especially, why 90mm? Anything less than 100mm in a wall leaf is usually frowned on.
I inadvertently bought some grr...grrr...
Anyone know?
TIA for any info
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http://www.plasmor.co.uk/html/products/plasmor-product-selector/concrete-blocks/lightweight-aggregate-blocks/aglite/?PHPSESSID f648add4e125646c033e7d91fcfcf4
Maybe it's 'effin safely' standard 18" x 9" x 4" dense concrete blocks might be too heavy for brickies .... I humped cwt bags of cement about on summer holidays when working for Building companies ... now it's considered too heavy to lift and 25Kg baby bags are the norm.
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On Wed, 24 Mar 2010 13:35:19 -0000, Rick Hughes wrote:

Probably needed doing. My Brother used to carry 200lb sacks of beans, throw hay bales up 6 layers etc. when working on a farm. When he worked in removals, he lifted a heavy, 3/4-seater settee by one end to give it to the 2 blokes at the top of some steps. He now suffers from arthritis and investigation has shown that most of his joints are worn out. I'm 3 years older and have no arthritis - the advantage of light mechanical and chemical work.
--
Peter.
2x4 - thick plank; 4x4 - two of 'em.
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Rick Hughes wrote:

Max parcel weight is 35Kg on most things.
We are a nation of wusses these days.
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1cwt used to be such a standard load ......... 25kg ? .... I blame the EEC, never would have happened when the Atlas was pink.
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Its to create spacing of 400mm and 200mm, supposedly reduces cutting, presumably when its all blocks and no bricks. Also, as others pointed out, reduced weight. I bought some standard sized blocks, and noticed they were all about 95mm. Was quite obvious when laying in with some bricks, 102.5mm. Simon.
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