Xilinx 7-Series architecture utilizes a hierarchical design of chainable structures to scale across the Spartan, Artix, Kintex, and Virtex product lines. This documentation focuses on the Artix and Kintex devices and omits some concepts introduced in Virtex devices.
At the top-level, 7-Series devices are divided into two halves by a virtual horizontal line separating two sets of global clock buffers (BUFGs). While global clocks can be connected such that they span both sets of BUFGs, the two halves defined by this division are treated as separate entities as related to configuration. The halves are referred to simply as the top and bottom halves.
Each half is next divided vertically into one or more horizontal clock rows, numbered outward from the global clock buffer dividing line. Each horizontal clock row contains 12 clock lines that extend across the device perpendicular to the global clock spine. Similar to the global clock spine, each horizontal clock row is divided into two halves by two sets of horizontal clock buffers (BUFHs), one on each side of the global clock spine, yielding two clock domains. Horizontal clocks may be used within a single clock domain, connected to span both clock domains in a horizontal clock row, or connected to global clocks.