When designing the letter Đ (uppercase) and đ (lowercase), the thickness, length, and position of the cross bar play a crucial role in legibility. If the bar is too thin, legibility is compromised at small sizes. To avoid kerning issues, the length of the bar must not extend too far from its base. In a serif typeface, the bar can only exceed slightly beyond its serif. For the letter Đ, the bar needs to be shorter on the left and longer on the right. In contrast, the bar on the letter đ needs to be longer on the left and shorter on the right.

In Vietnamese, the bar on the letter Đ must be positioned in the middle of the cap height. The Vietnamese writing system has no overdot, palatal hook, macron, underdot or undercomma with the letter D.

In the following example, the third uppercase Đ (right) from Alegreya Sans had the bar placed above, which was incorrect. I reached out to Juan Pablo del Peral who designed the Alegreya family and he made the update, which is the middle Đ in the illustration.