Voting is underway on Wednesday in by-elections across West Bengal, Tamil Nadu, Puducherry and Assam. But what exactly is a by-election, and why does it matter?

A by-election is held to fill a seat that falls vacant between general elections. This happens when a lawmaker resigns, passes away or is disqualified. Voters in that constituency choose a new representative for the remaining term.

Though only a few seats are involved, by-elections carry weight. They can change the numbers in a state assembly, especially where the ruling side has a thin majority.

They are also read as a report card on governments. Parties test their popularity, and the results often set the tone for the next full election.

Counting will show whether ruling parties held their seats or the opposition made inroads. Either way, the outcome will shape political strategy in these states in the months ahead.

This round covers constituencies in the east, south and northeast, plus the union territory of Puducherry. Each contest has its own local issues, from development work to candidate selection.