A painting knife is an artist's tool with a flexible steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 blade used to apply paint on the canvas.

It has a pointed tip, lowered or "cranked" like a trowel

Trowel

A trowel is one of several similar hand tools used for digging, smoothing, or otherwise moving around small amounts of viscous or particulate material.-Hand tools:...

, suited for painting on canvas.
The blade can be of different lengths and shapes : triangular, rectangular or more diamond like.

A painting knife is different from a palette knife

Palette knife

A palette knife is a blunt tool used for mixing or applying paint, with a flexible steel blade. It is primarily used for mixing paint colors, paste, etc., or for marbling, decorative endpapers, etc...

 which has a straight wide blade and a rounded tip, better suited for mixing paints on the palette.

Knife painters

Famous knife painters are : Titian

Titian

Tiziano Vecelli or Tiziano Vecellio Tiziano Vecelli or Tiziano Vecellio Tiziano Vecelli or Tiziano Vecellio (c. 1488/1490 – 27 August 1576 better known as Titian was an Italian painter, the most important member of the 16th-century Venetian school. He was born in Pieve di Cadore, near...

, Franz Hals, Rembrandt, Fragonard, Courbet

Courbet

Courbet may refer toPeople* Amédée Courbet , a French admiral* Félicien Courbet , a Belgian water polo player and breaststroke swimmer who competed in the 1908 Summer Olympics* Gustave Courbet , a French painterships...

, Impressionnists

Impressionism

Impressionism was a 19th-century art movement that originated with a group of Paris-based artists whose independent exhibitions brought them to prominence during the 1870s and 1880s...

, Nicolas de Staël

Nicolas de Staël

Nicolas de Staël was a painter known for his use of a thick impasto and his highly abstract landscape painting...

, Riopelle

Riopelle

Riopelle may refer to:* Jean-Paul Riopelle - painter and sculptor from Quebec, Canada* Jerry Riopelle - musician and record producer from the United States*Howard Riopelle , Canadian ice-hockey player...

, and Dirks

Dirks

Dirks, as a person, may refer to:*Jerald F. Dirks, an American author*Nicholas Dirks, an American academic*Rudolph Dirks , a German American cartoonistDirks may also refer to:...

.