There are four main muscles in the Beef Knuckle.

1. Rectus femoris
2. Vastus intermedius
3. Vastus lateralis
4. Vastus medialis.

The rectus femoris, vastus lateralis and vastus intermedius, known to customers in the USA as the tip center, tip side and tip bottom.

I mention them here for completeness, but in the real world, we use everyday terms. 

Mind you, each country has different names for these muscles so it can get kinda confusing after a while. I’ll use photographs to give a better understanding of what it is we are talking about.