Interlacing divides the frame into two fields, each containing half the number of lines.
With interlaced systems there are an odd number of lines in each frame.
Selecting a consistent field dominance in vision switching and linear editing systems will maintain color framing synchronization.
In 625/50 systems field one line one starts with line sync being coincident with the falling edge of the first broad pulse in the analogue field sync.
Digital formats use a single-bit in their Timing Reference Sequence as the "Field" flag.