Measuring & Layout

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A straight aluminium or wood body with spirit vials set at horizontal, vertical and often 45 degrees. A 4-foot level covers walls, doors and studs; a 2-foot fits in a bag. Check any level before trusting it - set it down, read it, flip it end for end, and it should read the same.
Buying tip
Buy a 4 ft for walls and doors. Check it against a known flat surface before you leave the store.