subject verb agreement

[user_gary]

The horse and carriage is at the door.
Bread and butter is his only food.
His secretary and his typist are on leave today.

Why `is' in the first and second sentences? Why not `are'?

[svartnik]

The horse and carriage is at the door.
Bread and butter is his only food.
His secretary and his typist are on leave today.

Why `is' in the first and second sentences? Why not `are'?

Because the compound subject is viewed as one thing