ThinkPad i series
The i series was developed as a low-cost machine line, with the intent of bringing even more affordable ThinkPads to the market. They were generally cheaper than the 300 series, which were already much cheaper than the others.
Strangely, some of the models were duplicates of the A series, X series, 200 series, and the 500 series, but often with much less features and lower performance. Some of the other models that were not merely duplicates of a certain series, had interesting features, such as being able to play music CDs while not being powered-on. Although, that came with a trade-off; no dock connectors.