Alvar Aalto Stool ????  

Not Aalto
It's a little hard to tell from your photo of the leg bend, but there are other features that point to it being not Aalto. If the entire leg is plywood, then that is a tell-tale sign.
The top also looks like it's plywood?

Patented method
Best way to identify Aalto production is by the patented way that the wood was bent to form the legs.

Thanks.
Thanks for the diagram. I understand about the legs now. Any ideas on who made my stool then? Ikea?

Might be
Might be ikea or someone similar.
One of my back burner projects is to do a identification guide for the various mid-century copies of this stool. Right off the top of my head I can think of at least half a dozen producers of something which looked remarkably like a A Aalto stool in the period from 1936 - 1969 each one with its own subtle differences. Not to mention the differences between the Finnish, Swedish and American production of the stools.