Horizontal traceability shows relationship among related items such as between requirements itself. It traces dependent items within a development phase. Vertical traceability is a characteristic identifying the source of requirements typically from requirements to design, to the source code and to test cases.
Horizontal traceability is an aspect identifying non hierarchical similarities, mutual properties, interactions, etc. among requirements and work products. For example assume we want to implement a login function in four different types of browsers. There are four sub teams to do this. Here the functional requirement remains the same. If any change in requirement happens, then it needs to be reflected across all the four browsers. These kind of dependent requirements are easily traceable if