Most letters written in a minuscule script are confined between two lines on the page, but some letters -- especially the b, d, and h -- may contain a vertical stroke that rises above the line. This stroke is called the ascender. A thickened vertical stroke may be described as a clubbed ascender because of its shape. The size, appearance, shape, or direction of the ascenders can help classify the script.
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Raymond Clemens and Timothy Graham, Introduction to Manuscript Studies (Ithaca, NY: Cornell University Press, 2007), 263.
Geoffrey Ashall Glaister. Encyclopedia of the Book, 2nd ed. (New Castle, DE: Oak Knoll Press; London: The British Library, 1996), 20.