Sash Windows work as well as they do thanks to the counter weights (which are hidden from view).
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The counter balance weights for sash windows are usually made out of lead or cast iron. Lead is much more dense than cast iron, so a sash weight made of lead will take up less space.
Each sash window should be counter balanced by its weights. So a sash (including the weight of the glass) that weighs 10kg should have two 5kg weights.
For dual hung sash windows the upper sash should have its counter weights being around 1 pound (0.45kg) heavier than the sash weight (so it will drop down). The lower sash should be around 1 pound lighter than the sash.
But – to make your life easier there are calculators online which you can rely upon to find the exact weight of sash counter weight!