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Reference.org
Annulus (firestop)
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<p>The annulus, or annular space, is the space between a <a href="/facts/Penetrant_(mechanical%2c_electrical%2c_or_structural)/5j7AJK29">penetrant</a> and anything that surrounds it, such as the sides of an opening or a <a href="/facts/Sleeve_(construction)/gLzjQlxU">sleeve</a>, depending on the case. It is named after the corresponding <a href="/facts/Annulus_(mathematics)/rhxKuoem">geometric concept</a> due to its shape. </p>