The reason a bar pendulum is called a compound pendulum:
The compound pendulum has an extended mass like a swinging bar.
The simplest form of the compound pendulum is a bar pendulum.
A bar pendulum with holes drilled along its length in equal parts is in the form of a rectangular bar.
A metallic bar that is one meter long makes up the bar pendulum. A sequence of circular holes, each measuring about 5 mm in diameter, are bored down the length of the bar.
Any hole can accommodate a horizontal knife edge, which suspends the bar.