Sephardim have a Dalet shaped kesher. Among Teimanim there are 2 minhagim- some have a single dalet and some a double dalet. Non Chassidic Ashkenazim use a single dalet, other than yekkis, who have a double dalet. Most Polish and Russian Chassidim have a single dalet while most Hungarian, Romanian and Czechoslovakian chassidim have a double dalet.
There is extensive discussion as to which is the correct kesher. I heard that 2 seforim recently came out specifically discussing this matter, each one representing one of the sides. For the most part, each person should stick with his minhag. However what do those who have no minhag do? Rav Yaakov Meir Stern brings sources to show that the double dalet is the correct kesher and hence one without a minhag should do so. However, Rav Dovid Morganstern in Zichron Eliyahu concludes that the single dalet is the correct kesher. (It is generally accepted that this sefer follows the psak of Rav Elyashiv unless noted otherwise.) The same opinion is concluded in Hadar Tefillin by Rav Raphael Toledano. He brings down Rav Shraya Diblitzky, who goes so far as to say that than the Dalet is the correct kesher and one who switches from his minhag of a double dalet to a single dalet brings bracha upon himself! Of course, one must consult his rav if he is without a minhag or has concerns about his minhag.