postgres equivalent of sql server begin transaction

Comparing Transactions between SQL Server and Postgres.

SQL Server : : BEGIN Transaction  -- Commit Transaction – Rollback Transaction

Postgres      : Begin – Commit – Rollback

Unlike SQL Server , Postgres commits or Rollback the transactions at the First Commit/Rollback comman after any begin statement  and Ignore Subsequent Commit/Rollbacks.

But in SQL Server , Each Begin Transaction requires a Commit/Rollback.