宋代的文書制度，除了下行文書和上行文書以外，還存在應當被視為平行文書的牒、關、諮報這類的文書。平行文書基本上是「非相統攝」的官府之間往來的文書。不過，正如從「牒」的「牒上」、「故牒」的幾個例子顯示，某些雖然具有「相統攝」關係卻不適合使用「符」、「帖」、「申狀」的場合，同樣也使用「牒」這種形式的文書，由此可見，宋代官府間的相互關係中還存在應當被稱為「內」、「外」兩種類型的命令系統。因此，宋代的命令系統並非一元的、金字塔層級式的構造，而是在中央，同時存在對以三省六部為中心的官僚機構進行牽制、監視的另一個系統的官僚機構的二元體制；在地方，則存在路—府州軍監—縣相互監視的構造。

In addition to official correspondences to either lower or higher bureaucratic levels in the Song dynasty, there were also parallel correspondences, such as die牒, guan關, and zibao諮報. These parallel correspondences were documents sent between administratively-unrelated governmental offices. Occasionally, different offices that were administratively related used die (such as in dieshang牒上 and gudie故牒) to replace more common appellations that implied hierarchy, such as fu符, tie帖, or shenzhuang申狀. This indicates that there was an “inner” and “outer” system of command in place that differentiated between various administratively-related offices. In addition, the official correspondence system reveals that the system of command in the Song dynasty was not hierarchical or pyramidal; the central government had a dual system composed, on the one hand, of the bureaucracy of the Three Departments and Six Ministries 三省六部 and, on the other hand, another bureaucracy, which oversaw the first. Local governments also had a system of checks and balances among circuits 路, prefectures 府州軍監 and districts 縣.