(This is a followup to the answers of Is ‘Th.’ in a name a short form of Theodor?, hoping to learn more details than are given there.)
It’s quite common in German for first-name initials to be given as multiple consonants, not just a single letter: e.g. Th. for Theodor, Thomas, etc., Chr. for Christian, Ph. for Philipp, and so on.
What are the rules/conventions/connotations for this? Is it seen as formal/old-fashioned, or is not doing it considered as markedly sloppy/anglicised? When using it, does it apply to certain traditional names only, or to all names with suitable beginnings? Is there a consistent rule of whether it applies just to di-/tri-graphs (e.g. Ch., Ph.) or to all consonant clusters (including also Chr., St., etc.), or certain particular clusters?
I’ve searched around a bit, but haven’t found anywhere properly discussing this convention. But my German is rather rusty, so at least for German-language sources, my search has been quite superficial.
(Of course, this isn’t unique to German; it’s used similarly in Dutch, and to some extent in many other languages. I’m interested primarily in its use in German, but sources describing the convention in Dutch would be better than nothing, insofar as Dutch and German typographical traditions have been quite closely linked historically.)