Hemd (Issue Shirt)
Members must own an issue-pattern white collarless Hemd. Additionally, members have the option of wearing a correct period civilian shirt, which may be added to the kit. A grey-wool collarless shirt which was trimmed in white was a common style of non-issue shirt used by soldiers.
One important function of the issue German shirt that is often overlooked in reproductions is that not only is it an issue shirt for your uniform, it also is designed to function as a night shirt and therefore it is pretty long - at least 18cm past the crotch (7-1/8" for you non-metric types). The sleeves should stop short of the wrist and should not be seen when wearing the tunic, even when arms are raised.
Photos from The German Army in the First World War: Uniforms and Equipment, 1914-1918
by Jurgen Kraus for the Bayerisches Armeemuseum.
The material should also not be linen, but light canvas/cotton drill for summer use, cotton jersey for winter use. The color should not be white but either off-white or natural, unbleached.
German veterans have also indicated that it was common practice to wear only a cotton union shirt beneath the uniform. Therefore, the wearing of a union suit is allowed - provided that any plastic buttons are replaced with appropriate period pattern buttons of bone, wood, or other correct materials. Remember, even if you choose to follow one of the shirt options, you must still own an issue shirt!