Leg: Basics
By Rio

Legs are the foundation of all humans - you stand on it every day. ;) Manga legs range from detailed and buff to generic, simplified lines. Simplified legs being the norm. Below is a row of legs in the front view. The first leg is typical, showing the simplified version I mentioned. Notice how the middle is practically straight and has no curves while the outer edge is just the opposite. The second example, on the other hand, has curves at both sides of the leg. Taking a closer look, the legs are shaped like cylinders with one end slightly wider than the other end from hip to knee, from knee to ankle.

Leg Front Views
At side and back view, the same idea applies. The back view of the leg tends to have dashes | /  \ | indicating the back of the knee. You can also do the same at the front using one or two dashes except reversed | \  / |.
Leg Side View Leg Back View Leg Miscellaneous Views
One important aspect about legs that you must keep in mind is when it is against another object whether if it's a chair or it's own self, the leg tends to widen out. The muscle gets pushed to the side or upwards depending on the situation. Above are three of such examples.

Top: Leg pushing up on itself.
Middle: Leg resting on chair.
Bottom: Leg against itself and on the floor.