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According to Duden (and all of my typographic handbooks), the following rules apply to abbreviations at the beginning of a sentence.

A simple abbreviation is written with a capital initial letter:

vgl. → Vgl. …

An abbreviation consisting of several parts should preferably be written out in full:

m. a. W. → Mit anderen Worten … (not: M. a. W.)

m. E. → Meines Erachtens ... (not: M. E.)

z. B. → Zum Beispiel ... (not: Z. B.)

d. h. → Das heißt … (not: D. Hh.)

The abbreviations i. A. (im Auftrag) and i. V. (in Vertretung) are written with a capital initial letter if they are used in front of an isolated signature. However, within a valediction or following the name of a company, they are written with a small initial letter:

Mit freundlichen Grüßen
i. V. Karl Müller

According to Duden (and all of my typographic handbooks), the following rules apply to abbreviations at the beginning of a sentence.

A simple abbreviation is written with a capital initial letter:

vgl. → Vgl. …

An abbreviation consisting of several parts should preferably be written out in full:

m. a. W. → Mit anderen Worten … (not: M. a. W.)

m. E. → Meines Erachtens ... (not: M. E.)

z. B. → Zum Beispiel ... (not: Z. B.)

d. h. → Das heißt … (not: D. H.)

The abbreviations i. A. (im Auftrag) and i. V. (in Vertretung) are written with a capital initial letter if they are used in front of an isolated signature. However, within a valediction or following the name of a company, they are written with a small initial letter:

Mit freundlichen Grüßen
i. V. Karl Müller

According to Duden (and all of my typographic handbooks), the following rules apply to abbreviations at the beginning of a sentence.

A simple abbreviation is written with a capital initial letter:

vgl. → Vgl. …

An abbreviation consisting of several parts should preferably be written out in full:

m. a. W. → Mit anderen Worten … (not: M. a. W.)

m. E. → Meines Erachtens ... (not: M. E.)

z. B. → Zum Beispiel ... (not: Z. B.)

d. h. → Das heißt … (not: D. h.)

The abbreviations i. A. (im Auftrag) and i. V. (in Vertretung) are written with a capital initial letter if they are used in front of an isolated signature. However, within a valediction or following the name of a company, they are written with a small initial letter:

Mit freundlichen Grüßen
i. V. Karl Müller

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source | link

According to Duden (and all of my typographic handbooks), the following rules apply to abbreviations at the beginning of a sentence.

A simple abbreviation is written with a capital initial letter:

vgl. → Vgl. …

An abbreviation consisting of several parts should preferably be written out in full:

m. a. W. → Mit anderen Worten … (not: M. a. W.)

m. E. → Meines Erachtens ... (not: M. E.)

z. B. → Zum Beispiel ... (not: Z. B.)

d. h. → Das heißt … (not: D. H.)

The abbreviations i. A. (im Auftrag) and i. V. (in Vertretung) are written with a capital initial letter if they are used in front of an isolated signature. However, within a valediction or following the name of a company, they are written with a small initial letter:

Mit freundlichen Grüßen
i. V. Karl Müller