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Circuit board assembly is mostly done in conveyorized flow lines in which each board visits each machine in a fixed sequence. It is common knowledge that balanced flow lines provide high throughput and low cost in high volume and low mix manufacturing. However, in flow lines, a single machine interruption stops the entire line when the buffer capacity is low. As a result, the throughput of circuit board assembly lines is often much lower than bottleneck machine throughput capacity. Lines with parallel machines can potentially reduce the effect of individual machine interruptions. This research is to quantify the throughput gain of parallel segments with analytical models. Simulation and a case study were also conducted to compare with the analytical results. Note to Practitioners-In theory, transfer lines or flow lines generate high throughput with low work-in-process. In reality, the benefit can be achieved only if the line is free of interruptions and is balanced. In most manufacturing systems, interruptions are common due to setups, material replenishments, errors and occasional machine breakdowns. An interrupted station can stop the entire line quickly in a transfer or flow line. In addition, in most modern manufacturing systems, the batch sizes become smaller and smaller. When the batches are very small (in extreme case, batch size of 1), it is almost impossible to achieve perfect balance when different jobs are processed in the same line. In both cases, the parallel segments in the line can increase the throughput without increasing the work-in-process. This paper presented the evidence via analytical models and simulation.