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Keystones have been used since antiquity at the apex of stone arches to lock the other stones into place. Additionally, wedge-shaped blocks, also called Keystones, have decorated architectural woodwork for centuries. In this capacity they are used not only with curved arches, but also with straight Door or Window Caps, as well as with cased openings.
When used with woodwork, Keystones typically extend slightly above the top of the adjoining mouldings (as shown above) and "stand proud", meaning their surface is slightly raised relative to the adjacent woodwork.
Most of our Polyurethane Keystones are designed to fit over specific Polyurethane Moulding profiles. This feature, incidentally, allows adjustment to the overall length of a Cap and/or Moulding, as the seam between the two sides can be hidden behind the Keystone. The use of Keystones can also avoid additional shipping charges on extra long Door & Window Caps because we can cut the Caps in half before shipping.
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