Final answer:
The inclusion of a larger number of wind instruments in orchestras emerged in the nineteenth century.
Step-by-step explanation:
The common element of orchestras today that emerged in the nineteenth century is the inclusion of a larger number of wind instruments. As improvements in the mechanics of woodwinds and brass instruments were made during the Industrial Revolution, these instruments became easier to play and produced a bigger, fuller sound. Composers in the post-Romantic era also started incorporating more wind instruments in their compositions, making their parts more challenging and significant to the overall orchestral sound.