The early Qing military was rooted in the Eight Banners first developed by Nurhaci to organize Jurchen society beyond petty clan affiliations. There were eight banners in all, differentiated by color. The yellow, bordered yellow, and white banners were known as the "Upper Three Banners" and were under the direct command of the emperor. Only Manchus belonging to the Upper Three Banners, and selected Han Chinese who had passed the highest level of martial exams could serve as the emperor's personal bodyguards. The remaining Banners were known as the "Lower Five Banners". They were commanded by hereditary Manchu princes descended from Nurhachi's immediate family, known informally as the "Iron cap princes". Together they formed the ruling council of the Manchu nation as well as high command of the army. Nurhachi's son Hong Taiji expanded the system to include mirrored Mongol and Han Banners. After capturing Beijing in 1644, the relatively small Banner armies were further augmented by the Green Standard Army, made up of those Ming troops who had surrendered to the Qing, which eventually outnumbered Banner troops three to one. They maintained their Ming era organization and were led by a mix of Banner and Green Standard officers.