Triangular number 4 has a value of 10.

The modern Greek word "τετρακτύς" (teh-tra-KTEES) ≈ "tetractys", also called the tetractys of the decad, is a triangle shape of triangular number 4.

This pattern and number was considered special (mystical) by Pythagoras and his followers for their "secret" worship. Each row and number had a special meaning.

• 1 "μονάδα" (mo-NA-tha) ≈ "unit, monad" and refers to the supreme being and to unity.
• 2 "δύναμη" (THEE-na-mee) ≈ "power" or dyad, requiring two parts, such as action-reaction. The English word "dynamic" comes from this Greek word.
• 3 "αρμονία" (ar-mo-NEE-a) ≈ "harmony". The English word "harmony" comes from this Greek word.
• 4. "κόσμος" (KO-smos) ≈ "world, people, universe" or people in the world

The tetractys also symbolized the four classical elements.

• 1. "φωτιά" (fo-tee-A) ≈ "fire".
• 2. "αέρας" (a-EH-ras) ≈ "air".
• 3. "υγρά" (ee-GRA) ≈ "water". The English word "hydro" in "hydrogen" comes from this Greek word.
• 4. "γη" (ghee) ≈ "earth, land" . The "ge" in "geometry" comes from this Greek word.

The tetractys also symbolized the four dimensions of space.

• 1. point (one point, zero dimensions, point)
• 2. line (two points, one dimension, line)
• 3. plane (three points, two dimensions, triangle)
• 4. space (four points, three dimensions, tetrahedron)

The pattern of triangular number 4 is used for the setup of 10 bowling pins.