The civil ensign (also known as merchant flag or merchant ensign) is the national flag flown by civil ships (merchant ships and others, as opposed to military) to denote nationality. Countries may have a national flag for most purposes on land, a distinct civil ensign for non-military ships, and a naval ensign for the navy; sometimes two or all of these flags are identical.
In most countries there was originally no distinction between the flag for armed state ships (navy) and private owned, usually unarmed ships (merchant marine). Today many countries, including the United States and France, continue the practice of having a single national flag for all or most purposes.
The civil ensigns that are different from the "general" national flag, can be grouped into a number of categories.
Civil ensigns with the national flag in the canton/British Red Ensigns
Several countries use red flags with, in most cases, either the respective national flag or the Union Flag in the canton, patterned after the British Red Ensign. British overseas territories either fly the plain Red Ensign or a Red Ensign with the respective colonial arms in the fly. Saudi Arabia puts its national flag in the canton of an otherwise green flag (note that the Saudi Arabian flag is hoisted with the flagpole to its right, so the canton is in the upper right corner of the flag).
Civil ensigns consisting of the national flag with an additional emblem
Well-known examples are the Italian civil ensign showing the shield with the arms of the sea republics or the Polish civil ensign with the arms of Poland. Most of these emblems were added to distinguish the ensign from similar flags of other countries.
In several (Spain and much of Spanish Latin America, and some European) countries there are two main versions of the flag, a simpler one (usually a striped flag) and a more elaborate one with the national arms. The simpler one is used as civil ensign (and in most cases also as civil flag), whereas the version with the arms is mainly used by the government and the military. In Spain and Argentina the flag without the arms is only a variant for civil use; the national flag is used also as a civil ensign. In El Salvador, the civil ensign also differs from the national flag in the proportions.
Civil ensigns differing from the national flag in the proportions
Several former British colonies use 1:2 as a proportion for their ensigns and 3:5 for flags ashore, whereas Slovenia, Croatia and Hungary have it the other way around, with ensigns at 2:3 and flags ashore at 1:2. France is a special case: the overall proportion is the same, but the bands on the ensign differ in width slightly.