In 1911 a 40" jacket averaged 30 inches in length. The fronts were nearly all straight with only the slightest opening at the bottom. Sleeves became slimmer and and featured the four button cuff detail. Shoulders were slimmer, often suits were advertised as having no padding.
The three button jacket was still the favorite. Long rolling lapels started to come back in style (still commonly 2 3/4" wide).
By 1915 the effects of WWI on men's style became obvious. Jackets were advertised as featuring the "military high-wasited effect". This style echoed the serviceman's uniform and was particularly popular with those returning home. These men sought a comfortable suit similar to those donned while in the war. The high starched collar gave way to the soft collar and high button shoes were replaced by oxfords.
Vest trends included the "high-falutin vest", fitted and smart, many with long-rolling lapels. Also, the popularity of the wrist watch instead of the fob sometimes lead to no vest at all. All vests were cut high and seen well above the jacket opening.
In 1919 the waist-seam coat was introduced with one button and a straight front with squared or blunt corners.
The semipeg trousers of the previous decade were all but gone, replaced by those cut narrower at the waist and ending properly at the ankle. They were cuffed, and creased on both the front and back.
In sportswear the Norfolk jacket dominated the menswear of the teens. This was often coupled with knickers, heavy wool hose and a tweed cap. It was also often paired with white flannel trousers and a straw boater for summer evenings.
Click here for my research and renderings of the 1910's.