sed-

*s(w)ēd, the ablative singular of Proto-Indo-European *swé (“self”) lengthened under phrasal stress. Original form preserving the inherited /d/, which has been reinterpreted as hiatus-filler, but lost otherwise in all cases of long vowel preceding final /d/, such as other ablatives.

Doublet of sē as well as sed, where the vowel shortened (or never lengthened).

sēd-

1. Prevocalic form of sē-

   ‎sēd- + ‎itiō → ‎sēditiō

• Appears in just one base - cf. red- in reditiō.

• De Vaan, Michiel (2008) “sē; sē-, se-, sō-, so-”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, page 549