It's so odd it's narrow to the top and then fans out. I've been trying to figure this out for years. I usually trim the branches back and will soon again. Thank you.
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Based upon the leaves, bark and form the tree is probably a fastigiate cultivar of Populus nigra, the Black poplar. It may be a true Lombardy Poplar, Populus nigra 'Italica' or one of the crosses of this tree with Populus nigra subsp. betulifolia' - Populus nigra Plantierensis Group. The former being widely planted as shelterbelts for golf courses and orchards, before being superseded in popularity in cooler wetter climes by the latter. Another possibility is P. simonsii 'Fastigiata' which is a cultivar of a chinese species, but is rarer in cultivation, so less likely to be yours.
Growth throughout the tree is fastigiate - even at the top. The wider canopy is probably due to loss of the top part of the main trunk - quite possibly it was "topped" at some point. I would be slightly concerned about branches arising from this point shedding as this sort of growth can be quite weak where it joins the main trunk. You may wish to take it back to the original topping point (looks to be about eaves level) which may result in a more balanced looking tree.