Flower of the week: Penstemon Riding Hood ‘Delft Blue’

I took these photos before seven this morning, hoping that the early light would give a truer colour than bright sunshine. They still give no idea of the wonderful, intense purple-blue of the flowers. This is a new plant to me and one I’m very pleased with. I bought it at the nursery sale last year and it came through the winter, no problem. Unlike the penstemon I perhaps mistakenly put it next to, it has a very neat, compact habit, so will never flop. Not that the big one flops because I supported it early with one of my hazel wigwams. Penstemons are great garden plants. Flower all summer, drought resistant, easy to propagate. What’s not to like?

Query

Call me a boring old pedant, but. I've been constantly irritated in a book I'm reading by references to 'cannons'. Now I've just seen the same usage on the BBC news website in connection with the Waterloo commemorations. Isn't the plural of cannon, cannon? As in
Cannon to right of them,
Cannon to left of them, etc.?

If I'm wrong, I take it back.