Learn French: Cette tenue n'est ni unie ni rayée. - This outfit is neither plain nor striped.

The French equivalent of 'neither... nor...' is ne... ni... ni.... The ne goes before the conjugated verb, becoming n' here because est starts with a vowel. Then you place a ni directly in front of each option you are ruling out: ni unie and ni rayée.

Why do both unie and rayée end in an extra -e?

Both of these are adjectives describing cette tenue (this outfit). Because tenue is a feminine noun, the adjectives must be in their feminine forms. We add an -e to the base masculine forms uni and rayé.

I thought uni meant 'united'. Why does it mean 'plain' here?

You are right that it means 'united' in contexts like Les États-Unis (The United States). However, when talking about clothing, fabrics, or colors, uni means 'solid-colored' or 'plain' (meaning it has no pattern).

What if I wanted to list a third pattern it doesn't have? Do I just add another ni?

Exactly! You can chain as many options as you need with this structure. For example, if you wanted to add that it isn't plaid either, you would just say n'est ni unie, ni rayée, ni à carreaux.