D major
23 of Haydn's 104 symphonies are in D major, making it the most often used main key of his symphonies. A great number of Mozart's unnumbered symphonies are in D major, namely K. 66c, 81/73, 97/73m, 95/73n, 120/111a and 161/163/141a. The symphony came from the overture, and "D major was by far the most common key for overtures in the second half of the eighteenth century."