 In quantum field theory, a slash through a symbol, such as "ⱥ", is shorthand for "γμaμ", where "a" is a covariant four-vector, the "γμ" are the gamma matrices, and the repeated index "μ" is summed over according to the Einstein notation.In medieval manuscripts such as the Domesday Book, "strikethrough" of text with red ink often functions as highlighting similar to modern underline.The HTML presentational inline element for strikethrough is codice_1 or codice_2 This element was, however, deprecated in the 1999 HTML 4.01 standard, and replaced by the codice_3 tag, a semantic element representing deleted text, which user agents (typically web browsers) often render as a strikethrough.