They are correlative words. Correlatives in Esperanto are often found in pairs. Here are some examples. I learned in kindergarten. Milionuloj estas tiaj viroj kiajn blondulinoj preferas.
Millionaires are the kind of men whom blonds prefer.
Always when he comes there is trouble. The correlatives in Esperanto form either nine groups of five each or five groups of nine each , depending on how you choose to put them together. Here are two groups that have five each. In each of these groups all the correlatives share the same ending but have different beginnings. Here two groups that have nine.