[Grammar] preposition with windowsill

[constantinusphilo]

What preposition is usually used with 'windowsill'? I thought it was always 'on'. However, consider the following:

"... and there were flowers everywhere, roses and carnations and anemones, on his desk, on the table, in the windowsills."

The Secret History, Donna Tartt

Why 'in'?

 

[Skrej]

You do hear both.

I'm more likely to use 'in' if it's a wide sill with lots of depth to it. For example, I keep a lot of my houseplants in my windowsills, because my windows are unusually deep.

 

[Tarheel]

I would use"on" there, but obviously both are used.