Karl:

Achnatherum calamagrostis:

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They're two entirely different genuses. Karl is Calamagrostis x acutiflora. One has calamagrostis as it's Genus name, and one as it's Species name. Calamagrostis, loosely, means reed-grass.

I wouldn't seem right if I didn't chime in with a comment on my namesake!
Swanoir, The two grasses are similar in three respects;
In bloom time ~ mid to late June in my zone.
In all season interest. Both these grasses have beautiful early blooms which dry of to a very attractive look and the plants & the blooms stand up well right on through the winter.
They are both cool season grasses.
They differ in height and in form. C. Karl Foerster is stiffly upright ~ rigid and about 5 feet tall in bloom. A. calamagrostis has a softer vase shaped form and is only about 3 feet tall. Both make a wonderful addition to any landscape.
Donn,
Interesting that your grasses plumes are so sparse. What happened?

A, that isn't mine. It's just a pic I snatched off the web. I don't have either of these, yet.

Thank you both for your reply. This makes a lot more sense to me now.