Silver buttons, belts, and ornaments now made and worn by the hill peoples in Southeast Asia have behind them a very ancient tradition of workmanship. The same artistic tradition is found in textile designs. Music, dance, and song were originally associated with tribal rituals.
I'm guessing you mean in Photoshop so you would 1. click the ruler tool 2. drag it along the line you know is straight 3. on the options bar, click straighten