A putty knife is a specialized tool used when glazing single glazed windows, to work putty

Putty

Putty is a generic term for a plastic material similar in texture to clay or dough typically used in domestic construction and repair as a sealant or filler. Painter's Putty is typically a linseed oil based product used for filling holes, minor cracks and defacements in wood only...

 around the edges of each pane of glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

. An experienced glazer will apply the putty by hand, and then smooth it with the knife. Modern insulated glazing

Insulated glazing

Insulated glazing also known as double glazing are double or triple glass window panes separated by an air or other gas filled space to reduce heat transfer across a part of the building envelope....

 may use other ways of securing the glass to the window frame.

A spackle

Spackling paste

Spackling paste is typically used to fill holes, small cracks and other minor surface defects in wood, drywall, and plaster. Three versions of spackling paste are currently on today's market: 1. regular paste ; 2. lightweight; 3...

 knife (called a scraper in British English

British English

British English, or English , is the broad term used to distinguish the forms of the English language used in the United Kingdom from forms used elsewhere...

, also known as a spatula in American English

American English

American English is a set of dialects of the English language used mostly in the United States. Approximately two-thirds of the world's native speakers of English live in the United States....

) is also commonly called a "putty knife", and is used for scraping surfaces or spreading material such as plaster

Plaster

Plaster is a building material used for coating walls and ceilings. Plaster starts as a dry powder similar to mortar or cement and like those materials it is mixed with water to form a paste which liberates heat and then hardens. Unlike mortar and cement, plaster remains quite soft after setting,...

 in various construction

Construction

In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...

 trades. Widths from 1" to 5" or 6" are commonly available. Wider-bladed knives

Taping knife

A taping knife or joint knife is a drywall tool with a wide blade for spreading joint compound, also known as "mud". It can be used to spread mud over nail and screw indents in new drywall applications and is also used when using paper or fiberglass drywall tape to cover seams...

 up to about 12" are used for sheet rocking

Drywall

Drywall, also known as plasterboard, wallboard or gypsum board is a panel made of gypsum plaster pressed between two thick sheets of paper...

. Larger blades are made, but generally lack the stability of the smaller blades and do not make a perfectly flat surface.

Stiff-blade knives, typically 1 mm or .040" thick, are suitable for scraping. Flexible-blade knives, typically 0,5 mm or .020" thick, are suitable for spreading. Disposable knives, with handle and blade molded as a single piece of plastic, are suitable for occasional jobs such as spreading roof patching tar, where cleanup may involve hazardous solvents.