The triangle, circle and square are the symbolic signs of Aikido.
TRIANGLE This is considered as a symbol of procreation.
One splits into two. A spot of Ki ( nothing ) is divided into two Ki's - positive and negative. The two Ki's interact and give birth to all things in the universe. It is also your triangular stance.
CIRCLE. This signifies a form of harmony attainable after bringing your partner under control.
SQUARE The four corners represent the characteristics of gas, liquid, flexible matter, and solid matter. These characteristics also allude to training methods. Ki, flowing, flexible, and solid. "Ki" and "Flowing" methods are for most purposes treated as one.
The training methods are: -
a) Gou-No-Keiko to practice hard and full strength mostly by basic technique.
b) Jyu-No-Keiko to practice softly with the principle of non-resistance but still practicing attack and defence.
c) Ryu-No-Keiko to practice very softly as running water and there will be no attack and no defence. Nothing will be opposed between opponents and partners by perfect harmony and peace. This style of practice is only for high Dan Grades.
d) Whilst the circle indicates circular movement, the triangular stance equates to body position and if you put two triangles together, you can get the square, which indicates a state of control.
e) exercise in Koto-Dama (Word Spirit), which was studied by the founder.
Adapted from Saito and others by Andrew Published 1986.