Kilometer km, meter: m, decimeter dm, centimeter: cm, millimeter: mm, micron:? M, nanometer: nm, nautical mile: 1852m, second: s, minute: min, hour: h, kg: kg, gram: g, mg: mg, microgram:? G, nak: ng, degree, pi, area unit: m? Volume volume: m? , sum: ∑, equal sign: =, unequal sign ≦, less than or equal to ≤, greater than or equal to ≥, approximately equal to matter, constant equal to ≦, parallel ∨, less than or equal to ≦, greater than or equal to ≧, congruence ≦, similarity: ∽. There are (SAS) edge, (SSS) edge and (ASA) edge behind congruent triangles's proof.