The first openly all-professional team was the famous Cincinnati Red Stockings of 1869-1870. They began as an amateur organization in 1866. But after the American Civil War, interest in baseball returned and the Red Stockings gained popularity as they went on road tour, known as "barnstorming." During their 1869 - 1870 "season" they went undefeated.
Nevertheless, they weren't making a profit and they were disbanded. But when the first professional league, the National Association of Professional Base Ball Players, began in 1871, the Cincinnati Red Stockings regrouped in Boston, joined the new National Association and formed the Boston Red Stockings. (They eventually evolved into the Atlanta Braves.)
The influence of the Red Stockings is still felt in that nearly every professional team in Cincinnati since then has worn red as their primary trim color.
The National Association folded with the better teams and some new additions regrouping as the National League in 1876. One new teams was the Cincinnati Red Stockings who later was expelled from the league in 1880 for selling beer at games and playing games on Sundays.
A separate American Association was created to challenge (but not play against) the National League. The AA was considered the low class version because it had cheaper admission prices, offered alcoholic beverages, and had teams in working class "river towns" like Pittsburgh, Cincinnati, Louisville and St. Louis.
A resurrected Cincinnati Red Stockings eventually shortened to the Cincinnati Reds and won the first American Association pennant. In 1890, the Reds were readmitted to the National League, and continue to play in Cincinnati to this day.
In the 1950s, "Reds" became a synonym for "Communist"and they officially changed their name to the "Cincinnati Redlegs" with the logo altered from 1956 to 1960 to remove the term "REDS" from the inside of the C symbol. "Reds" came back in 1961 uniforms.
Cincinnati Reds Fan Shop
When Charles Comiskey brought his St. Paul Saints team into Chicago in 1900, an older team name of the White Stockings which was quickly shortened to White Sox by the press. In 1912, the team started wearing the first incarnation of its "SOX" logo on the shirts.
The team is often called the "Chisox" in headlines to distinguish from the other sox - the "Bosox" of Boston (although within their own hometowns they are often just "the Sox."
Chicago White Sox Fan Shop
In 1901, the American League club in Boston spent 7 seasons wearing blue stockings and was just known as "Boston", the "Americans" or the "Boston Americans" (because they were in the American League and there was another National League Boston team).
For the 1908 season, the AL team shirts featured a red stocking across the front labeled "BOSTON" along with red stockings and white caps. Confusingly, the NL team also wore red stockings and red caps with an old-English "B".
The now familiar "RED SOX" first appeared in 1912, coincident with the opening of Fenway Park.
Boston Red Sox Fan Shop