Learn Finnish: Suomen lippu on suorakulmio. - The flag of Finland is a rectangle.

It is a compound word built from suora (straight) and kulma (angle or corner). The -io suffix is commonly used to create nouns for shapes, just like in kolmio (triangle) which comes from kolme (three).

Doesn't lippu usually mean ticket?

Lippu is a word with two very common meanings in Finnish: a ticket (like a bus or movie ticket) and a flag. You simply have to rely on the context to know which one is meant.

Why is suorakulmio in the basic nominative form instead of the partitive case?

When using the verb olla (to be), the describing noun stays in the nominative if it is a singular, countable thing. Since a rectangle is a single, complete shape, we use the basic form suorakulmio. If we were describing an uncountable substance, we would use the partitive case.

Why does the sentence use Suomen lippu instead of suomalainen lippu?

Suomen lippu literally means Finland's flag, referring specifically to the official national flag. If you said suomalainen lippu, it would just mean any flag that happens to be Finnish, such as one manufactured in Finland.