The flag of Switzerland consists of a red flag with a white cross (a bold, equilateral cross).
It is one of only two square sovereign-state flags, the other being the flag of the Vatican City. (The civil and state ensign, used by Swiss ships and boats, has more traditional proportions of 2:3.)
Only the dimensions of the cross are formally established since 1889: “The coat of arms of the federation is, within a red field, an upright white cross, whose [four] arms of equal length are a sixth longer than their width.” The size of the cross in relation to the field is not formally established except on the naval ensign, for which the ratio of the size of the cross to the height is 5:8, and to the length is 5:12. A ratio of 2:3 or 7:10 to the span of the flag is usual.
Use of the white cross as a military ensign (attached to the cantonal flags in the form of strips of linen) has been used in the Old Swiss Confederacy since the 14th century, but the modern design of a white cross suspended in a square red field was introduced only after the end of the Napoleonic period, in 1815, and was introduced as official national flag of Switzerland in 1889. Wikipedia