Consonant gradation is a phonological phenomenon where the consonants of a word change depending on its grammatical form or context. This often occurs in languages like Finnish and Estonian, where the initial consonant of a word may become softer or harder based on its usage in sentences. In Finnish, for example, the word for "book" is "kirja," but in the plural form, it changes to "kirjat." These changes can affect the meaning and grammatical function of words, making consonant gradation an important aspect of understanding and using these languages correctly.