Why is vinyl replacement window half inch smaller than measurement??

Hello,
I want to replace my old aluminum single panes with vinyl double hungs.
I pulled the inside molding off to reveal a rough opening of 36.75 wide, by 60.25 high.
I read that the windows (Certainteed bryn marw II)will be shipped half an inch smaller than the rough opening.... WHY? AND what do I fill the have inch gap with? Do I just sandwich a 1/4 inch board on each side of the rough opening and window?
thanks for any and all help! Keith
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Rough openings are just that-rough. The 1/4 " clearance is so you can plumb the window...
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And as the other poster pointed out, the expanders will cover that...
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"Keith" wantd to know, so he asked:

The plastic holders that go inside the opening (and hold the ends) will take up that space.
J.T.C.
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"Keith" wrote in message

If you want exact size, you must specify HTS (hold to size). Do yourself a favor and don't order exact size, rough openings are just that, rough. The play gives you room to compensate for out-of-square openings. Specify for a header expander, you may want to use this. It slips over the head of the window before installing, but, make sure you lay fiberglass insulation on the head of the window before slipping the expander over the window.
For those side areas, pack with insulation, but not to tight to bow frames (center window in opening). Then caulk, and you can make a simple 1/2 round to cover any gap, or really get into it and do some custom work with sill and apron etc.
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