In 1865 he sold his share of the Brunswick business to Wilhelm Grotrian and the company was renamed as Grotrian-Steinweg, Helfferich, Schulz, formerly Theodor Steinweg.

Theodore Steinway began a cooperative venture with the Mangeot brothers in Nancy, France, who for several years in the late 1860s imported harps and soundboards from Steinway & Sons in New York City, which they installed in their own piano cabinets and sold under the brand name "Mangeot-Steinway", mostly in France and England.

For shipping convenience, they decided upon a location in the major port city of Hamburg in Germany, where they opened a new Steinway & Sons factory in 1880.

The most important development is considered to be the single key mechanism fitted to the newly invented tubular frame built with tubes of brass which contain wooden sticks inside to allow accuracy and a simple screwing, patent gained 1871 for Steinway.

Since the Vienna mechanism disappeared at the beginning of the 20th century, all grand pianos have been built using the single key principle, mainly developed by C.F.

Also the reinstallation of the elements is supported with high accuracy on the one hand and on the other a quite simple adjustment to perfect working conditions.

Theodore Steinway's rim bending block patent of 1880, which is still used in every grand piano all over the world: long thin strips of sawn wood are glued together and clamped on a wing-shaped fixture with screwed pressing bars.

The previous method of casemaking for grands was much more expensive and time-consuming: it demanded the fitting together of wooden corner pieces and in particular the right-hand wall bent by steam into an S curve.

The steam bending process required very experienced workers, and a high proportion of the bent pieces had to be rejected.

Theodore Steinway's invention, which glued together thin saw-cut sheets was very economical: it permitted more cases to be made in a shorter time, with less loss of wood which had already been expensively dried over a period of years.

Theodore Steinway in Germany, the younger brothers often applied to the US patent office and obtained protection for these ideas.