What part of speech must a word be if it can intensified by adding the suffix -er or -est to it?

Adding "-er" or "-est" creates the comparative and superlative forms of adjectives, which are used to compare the degree of a quality.

For example:

* Tall (adjective) -> Taller (comparative) -> Tallest (superlative)

While some adverbs can also be comparative and superlative, they don't typically use these suffixes.