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Create cursor .....
>>Index on ... tag ...

will create an index in memory!
>
>Will it really? I am not sure about this, but I thought that perhaps VFP might decide from case to case (depending on size?) whether it is more appropriate to keep it in memory or on disk, just as it is supposed to do with the cursors themselves - or perhaps always put it on disk, with a temporary filename.

Yes, as long as you have enough RAM, just like a cursor will also stay in RAM until VFP run out of resources, then it will swap to your HD behind the scene, and more or less out of your control.