The eszett, or sharp s, is an integral part of the German language and its dialects. One of only a handful of letters worldwide with no orthographical majiscule form, the eszett is rendered ß, and may be capitalized as ẞ for use in all- and small-cap typography. When the character is unavailable, it may be substituted with 'ss' or 'sz', depending on situation.
The eszett is only natively found in the German language and its dialects, and as such, only German-language orthography rules apply to its use. Despite its ability to be substituted with 'ss', one should not consider the eszett a replacement for a double-s, only the other way around. The eszett is usually a distinct letter, and only replaces a double s when such is formed irregularly at the end of a word or word component. Words written with a double-s may not have such substituted with the eszett. Verfassung, for example, must never be written as Verfaßung. In addition, if an eszett is unavailable, one should always use 'sz' to replace the eszett if substituting in 'ss' would form a different word. For example, substituting 'ss' into the phrase in Maßen ("in limited amounts") would produce the phrase in Massen ("in massive amounts").
Unlike the eszett, the long s has no majiscule approximation, generally being rendered identically in all-caps, small-caps, and conventional typography—as ſ. Rules for its use are, by virtue of the two characters being alternative glyphs of each other, combined with rules for use of the short s (s).
English and related
The following rules apply to the use of the long and short s in English, Welsh, Cornish, and Lowland Scots:
A short s is used when in the majiscule form;
A short s is used terminally, except in abbreviations;
A short s is used before an apostrophe or a hyphen;
A short s is used before the letters 'f', 'b', and 'k';
A long s is used after a hyphen, the above-mentioned rules withstanding;
A short s is used after the letter 'f';
A short s is used after the letter 's', except after the second 's' in 'sss';
Compounds having the base stem ending in 'ss' and the trailing stem beginning with 's' may be hyphenated or not;
A long s is used initially and medially, the above-mentioned rules withstanding; and
A long s is used before a hyphen at a line break, even when a short s would be otherwise used in that medial position.