? (minuscule: ?), or Þ (thorn) with stroke was a scribal abbreviation common in the Middle Ages. It was used for Old English "þæt" (Modern English "that"), as well as Old Norse "þor-", the "-þan"/"-ðan" in síðan, "þat", "þæt", and "þess". In Old English texts, the stroke tended to be more slanted, while in Old Norse texts it was straight. In Middle English times, the ascender of the þ was reduced (making it similar to the Old English letter Wynn, ?), which caused the thorn with stroke abbreviation ( ) to be replaced with a thorn with a small t above the letter ( ).
Unicode encodes ? as Ꝥ LATIN CAPITAL LETTER THORN WITH STROKE (HTML
Ꝥ), and ? at ꝥ LATIN SMALL LETTER THORN WITH STROKE (HTML
A thorn with a stroke on the descender also exists. The codepoints are Ꝧ LATIN CAPITAL LETTER THORN WITH STROKE THROUGH DESCENDER (HTML
Ꝧ), and ꝧ LATIN SMALL LETTER THORN WITH STROKE THROUGH DESCENDER (HTML