第九等「五大夫」爵，於兩漢時期，一直是吏六石及高爵（官爵）的分界點。縱覽史料及相關學術研究，凡論及「軍功爵」者，多會對「五大夫」爵，投以極多的關注。然「五大夫」於先秦時期，便已被授予，於傳世文獻中，至少可見魏、秦、趙等三國，「五大夫」既是爵名，也是官名。在先秦，能領兵作戰的將領，多擁有「五大夫」的爵級，因「五大夫」是賞給對國家有極大功勞之人。商鞅在秦國的變法中，奠立了一套完整的十七級軍功爵制（在一級之下，還有「小夫」），其中「五大夫」位於第九等，比起第五等「大夫」爵（高爵的起點），又高出四等。因此，得爵「五大夫」者，除了可享有《商君書》所規範的一切高爵權益，另外還能免除徭役，食邑最多也可高達六百家。秦始皇統一天下後，「五大夫」的政治權益，雖多和統一前重疊，但秦時的領土，已是由偏處西陲而擴張至全國。史料及出土的簡牘中，雖未見秦統一後，又增多了那些權益，但至少在食邑上，恐將隨著地域的富庶程度，而得著更高的受益。此外，秦始皇二十八年（219BC），除了授躲避暴風雨的大樹為「五大夫」爵，還將陪同其出遊及參與議政的大臣，
從戰國時期的十級門檻，下降至第九等「五大夫」爵。秦始皇用皇權高舉了「五大夫」的尊寵形象，使「五大夫」因此而奠定了更崇高的政治地位，此舉或已深植人心，故至漢初（惠帝即位詔），便將「五大夫」規範為官爵的起點。

The rank wudaifu, a tier nine nobility, was the dividing line between
600-dan officials and high level nobility during the Han dynasty. A look
at historical data and related research shows that whenever noble ranks
for military merit are spoken of, most discussion centers on the ranks of
the wudaifu （or five senior officials）. However, this rank was already
being conferred during pre-Qin times—in our textus receptus we can see
that at least Wei, Qin, and Zhao had a noble （and official） rank called
wudaifu. During these pre-Qin times, generals who lead armies to war
often held this rank, as it was bestowed on those who made major
contributions to the state. In Shang Yang’s political reformations in Qin,
he established a 17-tiered rank system for military merit （below the first
rank was another rank called xiaofu）. In this system, wudaifu was listed
as tier nine, which places it four ranks above the tier five daifu （which
was the starting point for the high level ranks）. Therefore, those that
attained the rank wudaifu, in addition to the powers and authority given to the high level ranks （as delineated in the Book of Lord Shang）, were
exempt from corvée and their fief could be up to 600 households in size.
When the First Emperor of Qin unified China, the political rights of
wudaifu remained the same, with the exception being that now Qin
controlled the whole of China as opposed to just a small section in the
western borderlands. Historical records and unearthed bamboo strips both
show that although their rights and privileges stayed the same, their fiefs were increased in size. Also, in 219 BC, the First Emperor bestowed the rank of wudaifu on the tree than shielded them from the rainstorm, and at the same time he lowered the rank of all those accompanying him and his advisors from the 10th level common in the Warring States period to the 9th tier of wudaifu. The First Emperor used his authority as emperor to raise the prestigious image of the rank wudaifu, which paved the way for its elevated political status. This took root in the minds of nobility, so that in the beginning of the Han dynasty （when Emperor Hui took the throne）, wudaifu was tagged as the beginning rank of high nobility.