Đ, known as crossed D or dyet, is a letter formed from the base character D/d overlaid with a crossbar. Crossing was used to create eth (ð), but eth has an uncial as its base whereas đ is based on the straight-backed roman d, like in Sámi Languages and Vietnamese. Crossed d is a letter in the alphabets of several languages and is used in linguistics as a voiced dental fricative.
Sign with the letter Đ, in Oaxaca, Mexico
A page from the đ section of de Rhodes's Dictioniarum Annamiticum, a 1651 Vietnamese-Portuguese-Latin dictionary. As with the B with flourish, Đ only appears in lowercase in de Rhodes's works.
Eth, known as ðæt in Old English, is a letter used in Old English, Middle English, Icelandic, Faroese, and Elfdalian.
A sample of Icelandic handwriting with some instances of lowercase ð clearly visible: in the words Borðum, við and niður. Also visible is a thorn in the word því.