A spirit level or bubble level is an instrument

Measuring instrument

In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...

 designed to indicate whether a surface is
horizontal

Horizontal plane

In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...

 (level) or vertical

Vertical direction

In astronomy, geography, geometry and related sciences and contexts, a direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the gravity field, i.e., with the direction of the gravitational force at that point...

 (plumb

Plumb-bob

A plumb-bob or a plummet is a weight, usually with a pointed tip on the bottom, that is suspended from a string and used as a vertical reference line, or plumb-line....

). Different types of spirit levels may be used by carpenter

Carpenter

A carpenter is a skilled craftsperson who works with timber to construct, install and maintain buildings, furniture, and other objects. The work, known as carpentry, may involve manual labor and work outdoors....

s, stonemasons, bricklayer

Bricklayer

A bricklayer or mason is a craftsman who lays bricks to construct brickwork. The term also refers to personnel who use blocks to construct blockwork walls and other forms of masonry. In British and Australian English, a bricklayer is colloquially known as a "brickie".The training of a trade in...

s, other building trades workers, surveyors, millwright

Millwright

A millwright is a craftsman or tradesman engaged with the construction and maintenance of machinery.Early millwrights were specialist carpenters who erected machines used in agriculture, food processing and processing lumber and paper...

s and other metalworkers, and in some photographic

Photography

Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...

 or videographic

Videography

Videography refers to the process of capturing moving images on electronic media even streaming media). The term includes methods of video production and post-production...

 work.

Original spirit levels had two banana

Banana

Banana is the common name for herbaceous plants of the genus Musa and for the fruit they produce. Bananas come in a variety of sizes and colors when ripe, including yellow, purple, and red....

-shaped curved glass vial

Vial

A vial is a relatively small glass vessel or bottle, especially used to store medication as liquids, powders or in other forms like capsules. They can also be sample vessels; e.g., for use in autosampler devices in analytical chromatography.The glass can be colourless or coloured, clear or amber...

s at each viewing point and were much more complicated to use.