Actually, you can start to see rollover issues with fewer than 6 keys down. Consider if the keys are (from the perspective of the controller) wired in a 3x3 gr

Actually, you can start to see rollover issues with fewer than 6 keys down. Consider if the keys are (from the perspective of the controller) wired in a 3x3 grid: A B C D E F G H I Most controllers work by running a signal down the "rows" and reading back on the "columns" (or vice versa) to determine which switches are closed. If you hold down A and D, and then press either "B" or "E", without diodes, it looks like both B and E are pressed. Controllers try to mitigate this problem by locking out troublesome combinations, so you end up with worst-case of 2-key rollover. The logical key matrix may end up designed in a way very different from the physical layout, to avoid putting commonly-used-together keys in the same row or column. This does have non-gaming implications: commands like shift-control-alt-S (IIRC, Photoshop's "save as non-PSD" command) are blocked on some keyboards.