COVER (Page 251 DMG)
To determine whether a target has cover against an
attack or other effect on a grid, choose a corner of the
attacker’s space or the point of origin of an area of effect.
Then trace imaginary lines from that corner to every
corner of any one square the target occupies. If one or
two of those lines are blocked by an obstacle (including
another creature), the target ha half cover. If three or
four of those lines are blocked but the attack can still
reach the target (such as when the target is behind an
arrow slit), the target has three-quarters cover.
On hexes, use the same procedure as a grid, drawing
lines between the corners of the hexagons. The target
has half cover if up to three line are blocked by an
obstacle, and three-quarters cover if four or more lines
are blocked but the attack can still reach the target.