In contemporary English writing, is it appropriate to use the single character "æ" rather than the two character "ae"? If so, in what contexts should one be favoured over the other?

There’s no point anymore in English. It often creates a pretentious impression too.

Vowel ligatures like “Æ æ” (letter Ash, Ash Tree, Æsh, Aesh, Æsċ, Aesc) and “Œ œ” (letter Ethel, Œthel, Oethel, Ēðel, Odal) (with or without diacritics like ǣ, ǽ, æ̀, æ̂, æ̃) are just not used anymore in English typesetting or printing since the 1920s — so never mind even writing.

All are easier to type or write out as two letters (ae, oe, etc) anyway.

Indeed, late Middle English around the 1450s went back to the Classical Latin practice of writing the ash and the ethel as two separate letters (“ae” and “oe’). Vowel letters were only ever regularly ligatured during High Middle Ages (1000s–1300s).

Vowel ligatures were somewhat revived in Victorian times (1837–1901) as a typesetting or editorial affectation.

Today, vowel ligatures are only ever used for:—

specific terms or names (Ænglisc, æsc, œthel, Ingvæonic, Lætitia, etc)

or deliberate “special effects” (as in archæology, encyclopœdia, manœuvre, mediæval, etc),

or proper foreign spellings (such as in French, where they are true ligatures due to etymology: sœur, bœuf, etc).

In case anyone is wondering:—

The French æ is called “e dans l’a” (“E in the A”).

The French œ is called “e dans l’o” (“E in the O”) or “o et e collés” (“O and E glued”) or nicknamed “œufs dans l’eau” (“eggs in water”).

Thanks for the A2A.