In an effort to understand both the Battle of Cannae and its early use of double envelopment (Pincer movement) in 216 BC, I came across the following diagram:
The diagram doesn't seem particularly well labelled. I take it from context that Hannibal's forces are in blue and the Romans are red, but I am left with the following questions:
What are the red squares that enclose small red squares?
Am I correct to think that the sold blue semi-circle means that forces were left in the center of the front line, in addition to flanking on both sides, as opposed to other Pincer movements I've seen where 100% of forces are split between the right and left envelopments?
How did the Roman cavalry become separated and does their movement away from the battle indicate a retreat with pursuit by the Numidian cavalry?
Do we know if it was 215 or 216 BC? The diagram says 215 but the article says 216.
I can answer some of these questions, as I had a hand in writing the present version of the Wikipedia article -- which I admit needs more work.
The red squares are the Roman camps, as labelled in the other map @TomasBy links to.
Hannibal put his weaker infantry units in the middle of his line, in order to lure the Roman troops in, so they could be attacked from the sides by his stronger infantry units. (Robert O'Donnell's book, mentioned in the Wikipedia article, explains the battle quite well.)
The Carthaginian cavalry was far better than the Roman. (The Romans never figured out how to be good horsemen, & ended up recruiting from other nationalities for their cavalry.) The Carthaginians defeated the Roman cavalry, & chased the cavalry from the field.
216 BC is the correct date. (For some reason, the source of the map -- the US Military Academy -- got the year wrong.) What is in dispute is the actual date of the battle: our primary source states it was 2 August, but the pre-Julian calendar was known to have been only loosely accurate, & one scholar has calculated the actual date to have been 21 June. YMMV.