He is best known for the reintroduction of a Rundschrift (round script) style of calligraphy and the broad pen nib associated with it.

[1] Whilst ronde script rendered with quill originated in late 16th-century France and was widely used in the country throughout the 17th and 18th centuries, it fell out of popularity after the invention of the mass-produced pointed pen from steel in early 19th century.

Round writing was designed to be a visually appealing, standardized style of penmanship which was easy to learn and execute, and Soennecken published books on the topic in several languages.

[2][3] As a result, broad-nibbed pen (this time from steel) was popular again not just in Germany, but also in France, Russia and elsewhere; scribes of the French Ministry of Finance used it even until right after World War II.

In 1876 he and his company relocated to Poppelsdorf, near Bonn, to be closer to the University which later awarded him the honorary title Dr. med.