In linguistics, ablaut is a system of apophony (regular vowel variations) in Proto-Indo-European (PIE) and its far-reaching consequences in all of the modern Indo-European languages. An example of ablaut in English is the strong verb sing, sang, sung and its related noun song.

The term ablaut (from German ab- in the sense "down, reducing" + Laut "sound") was coined in the early nineteenth century by the linguist Jacob Grimm. However, the phenomenon itself was first observed more than 2,000 years earlier by the Sanskrit grammarians and codified by Pāṇini in his Ashtadhyayi, where the terms guṇa and vṛddhi were used to describe the phenomena now known as the full grade and lengthened grade, respectively. In the context of European languages, the phenomenon was first described in the early 18th century by the Dutch linguist Lambert ten Kate in his book Gemeenschap tussen de Gottische spraeke en de Nederduytsche ("Commonality between the Gothic language and Lower German (Dutch)", 1710).

Vowel gradation is any vowel difference between two related words (e.g. photograph [ˈfəʊtəgrɑːf] and photography [fəˈtɒgrəfi]) or two forms of the same word (e.g. man and men). The difference need not be indicated in the spelling. There are many kinds of vowel gradation in English and other languages, and these are discussed generally in the article apophony. Some involve a variation in vowel length (quantitative gradation: photograph and photography), others in vowel colouring (qualitative gradation: man/men), and others the complete disappearance of a vowel (reduction to zero: could not → couldn't).

The Indo-European languages are a family (or phylum) of several hundred related languages and dialects, including most major current languages of Europe, the Iranian plateau, and South Asia and also historically predominant in Anatolia. With written attestations appearing since the Bronze Age, in the form of the Anatolian languages and Mycenaean Greek, the Indo-European family is significant to the field of historical linguistics as possessing the longest recorded history after the Afroasiatic family.

Indo-European languages are spoken by almost three billion native speakers, the largest number for any recognised language family. Of the twenty languages with the largest numbers of native speakers according to SIL Ethnologue, twelve are Indo-European: Spanish, English, Hindi, Portuguese, Bengali, Russian, German, Marathi, French, Italian, Punjabi, and Urdu, accounting for over 1.7 billion native speakers. Several disputed proposals link Indo-European to other major language families.

Suggestions of similarities between Indian and European languages began to be made by European visitors to India in the 16th century. In 1583 Thomas Stephens, an English Jesuit missionary in Goa, noted similarities between Indian languages, specifically Konkani, and Greek and Latin. These observations, however, were included in a letter to his brother which was not published until the twentieth century.