The following text is an extract from the Eissler book.
Chapter 3
Eissler, Eißler, or Eiszler ?
The spelling of the names changed during the course of the centuries.
The very first recordings of persons of this name are found in the store books (in the old Wurttemberg region from approximately 1520 on) of the various towns. There, the following forms are found: Eißelin, Eißellin, Yßelin, Yßellin and Ißlin. These forms appear alongside each other. Apparently family names aleady existed by this time.
It is surprising that the ending -in (normally used as the ending of housewife’s name) is used for men too, which only appears sporadically in the time of parish registers (from approximately 1550 on). The initial Y-, which was common in some regions, also disappears in the course of a few decades.
This is probably the result of the Lutheran Reformation, which not only brought about the simplification of church rites (up to then mostly Roman Catholic) but also of the spelling. In spite of many remainders of dialects an almost uniform spelling, at least in Protestant regions, emerged which up to then mostly depended on the good hearing of the clergy-man or on the colorfull pronounciation of the citizens.
In the oldest parish registers in some towns (around 1550) the names Eisele, Eiseler or Eißeler appear which later changed to Eissler. Families with the current name Eisele are not considered in this book. In some regions the names Isler, Iseler or Ißler appear. These names have not been examined either even though a connection is possible.
By the end of the 16th century the parish registers decided on the spelling Eisseler or Eiseler but at the same time the spelling usual today of Eißler and Eissler also appeared. However, today the difference is blotted out once more because the printing media often exchanges the -ss- by -ß- and vice versa. This sometimes even happens within a family which can bring about curious situations. The name of a person appeared in the telephone book twice, once wirtten with -ss- and once with -ß-. The planed simplification of German spelling and the reform of the Law for maritial names will not be a big help either.
Only one branch of the family saved the spelling error of a pastor over the centuries and still writes the name Eisler.
In America the sign -ß- which does not exist in English disappeared and the name can be found spelled Eiszler or Eis(s)ler.
I know that many families want to retain spelling their names in a special way.
But to simplify matters I used the spelling EISSLER for all persons and have marked eventual deviations separately. For similar reasons todays forms of ancient spellings was used if they have not led up to independant family lines.