Two different and incompatible definitions of <a href="https://en.wikipedia.org/wiki/Airfoil" class="extiw" title="en:Airfoil">airfoil</a> thickness are used in the aerodynamics field (here illustrated on the same airfoil).
<ul><li>In  <b>case A</b> above the thickness is measured <i>perpendicular to the foil camber line</i>. This is sometimes described as the "American convention"<sup id="cite_ref-Houghton_1-0" class="reference"><a href="#cite_note-Houghton-1">[1]</a></sup> and can be seen in Houghton &amp; Carpenter 2003,<sup id="cite_ref-Houghton_1-1" class="reference"><a href="#cite_note-Houghton-1">[1]</a></sup> Phillips 2010.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2">[2]</a></sup></li></ul><ul><li>In <b>case B</b> above the thickness is measured <i>perpendicular to the foil chord line</i>. This is sometimes described as the "British convention" and can be seen in Bertin &amp; Cummings 2009<sup id="cite_ref-3" class="reference"><a href="#cite_note-3">[3]</a></sup>.</li></ul><p>Note that each definition implies a different camber line geometry for a given airfoil. Since the camber line curvature is small on most aircraft, the effect of difference between the definitions is often small.
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<ol class="references"><li id="cite_note-Houghton-1"><span class="mw-cite-backlink">↑ <a href="#cite_ref-Houghton_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Houghton_1-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><cite class="book" style="font-style:normal">Houghton, E. L.; Carpenter, P.W. (2003)   Butterworth Heinmann , ed.   <i> Aerodynamics for Engineering Students</i> (5<sup>th</sup> ed.)  <small><a href="https://en.wikipedia.org/wiki/International_Standard_Book_Number" class="extiw" title="en:International Standard Book Number">ISBN</a>:  <a href="//commons.wikimedia.org/wiki/Special:BookSources/0750651113" title="Special:BookSources/0750651113">0750651113</a>. </small></cite> p.17</span>
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<li id="cite_note-2"><span class="mw-cite-backlink"><a href="#cite_ref-2">↑</a></span> <span class="reference-text"><cite class="book" style="font-style:normal">Phillips, Warren F. (2010)      <i> Mechanics of Flight</i> (2<sup>nd</sup> ed.), Wiley &amp; Sons  <small><a href="https://en.wikipedia.org/wiki/International_Standard_Book_Number" class="extiw" title="en:International Standard Book Number">ISBN</a>:  <a href="//commons.wikimedia.org/wiki/Special:BookSources/9780470539750" title="Special:BookSources/9780470539750">9780470539750</a>. </small></cite> p.27</span>
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<li id="cite_note-3"><span class="mw-cite-backlink"><a href="#cite_ref-3">↑</a></span> <span class="reference-text"><cite class="book" style="font-style:normal">Bertin, John J.; Cummings, Russel M. (2009)   Pearson Prentice Hall , ed.   <i> Aerodynamics for Engineers</i> (5<sup>th</sup> ed.)  <small><a href="https://en.wikipedia.org/wiki/International_Standard_Book_Number" class="extiw" title="en:International Standard Book Number">ISBN</a>:  <a href="//commons.wikimedia.org/wiki/Special:BookSources/9780132272681" title="Special:BookSources/9780132272681">9780132272681</a>. </small></cite> p.199</span>
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<a href="//commons.wikimedia.org/wiki/User:Ariadacapo" title="User:Ariadacapo">Olivier Cleynen</a>
<span class="int-own-work" lang="en">Own work</span>