A characteristic of German ribbons wear was displaying certain decorations from a tunic buttonhole. Generally, any war decoration or lifesaving medal ribbon could be worn in this manner, normally in a "reduced wear" form of dress as displayed here by Rittmeister Beeckmann in 1928.
He has chosen to wear his highest decoration-- Prussia's Hohenzollern House Order 3rd CLass with swords-- as the ONLY ribbon in his second tunic buttonhole, without any ribbon bar at all. To be correct, when he wore his ribbon bar, either this ribbon was removed, or a ribbon bar without this Order would have been displayed.
Very often in WW2 photos, soldiers will be seen wearing buttonhole ribbons which are duplicated by the same awards on a ribbon bar--see Buttonhole Ribbons 04. That is simply a case of over-dressing for the photographer and was strictly non-regulation.
It was also traditional for the most recent decoration to be worn in a buttonhole in this way. That is why WW1 veterans will be seen with their 1914 Iron Cross 2nd Class on a ribbon bar but their WW2 KVK2 ribbon from their tunic front--see Buttonhole Ribbons 05.
Ribbon bars in the "traditional" 15mm width first came into use in 1915 because by that point officers were accumulating more ribbons than could be worn from a buttonhole. In the Second Wolrd War it was never fashionable to wear more than TWO ribbons from a buttonhole: usually the EK2 1914 with 1939 Spange or 1939 EK2, 1939 KVK2(X), and to highlight its special status, the 1941/42 "East Medal."
While not literally ribbon bars, buttonhole ribbons are directly related to their wear-- or lack therof.