The source of the below information is The Texas Handbook Online.
As you will see when you view the various census files, some overlap in the three counties occurs. In fact due to officials not knowing exactly where the Jeff Davis County line was until 1882, when the Railroad surveyed the area, the western part of Pecos County is included in Jeff Davis County enumerations. Specifically the area around Balmorea.
I have included a short history of the establishment of each county to try and explain why a researcher may find their ancestor in one county one year and in another the next.
Bexar County covered much of the western edge of settlement in Texas. During the late Mexican period, Texas had been divided into four departments, with the department of Bexar stretching from the Rio Grande to the Panhandle and as far west as El Paso. With the winning of Texas independence, the departments became counties, and on December 20, 1836, Bexar County was established, with San Antonio as county seat. Since 1860, when the partitioning of Bexar County began, 128 counties have been carved from the original county, leaving the present county at 1,248 square miles.
The United States census of 1850 reported no population for Presidio County; a sufficient number lived there to establish the county from Bexar Land District on January 3, 1850. Fort Leaton was the as the county seat. In 1854 the army built Fort Davis in northern Presidio County to protect travelers and settlers. By 1860 Indian attacks declined, and the census of that year recorded 574 whites, two free blacks, and four slaves. As in most frontier areas men outnumbered women 436 to 144. With the outbreak of the Civil War Fort Davis closed, and Indian attacks resumed. The fort was reopened in 1867, and the population of the county increased threefold by 1870, when 1,636 people were listed as residents, 494 of them were women and 772 were Mexican emigrants. The black population increased to 489 when buffalo soldiers were stationed at Fort Davis. Presidio County was organized in 1875 as the largest county in the United States, with 12,000 square miles. Fort Davis was named the county seat.
In 1887, Brewster County was marked off from Presidio County, as were Jeff Davis, Buchel, and Foley counties. Brewster County was named for Henry P. Brewster, secretary of war under David G. Burnet. Buchel and Foley counties were not organized and were attached to Brewster County for judicial purposes. The first Brewster County elections were held on February 4, 1887, when Murphyville was selected as county seat; on March 14 of that year a contract was let for the construction of the Brewster County courthouse and jail. In 1890 Brewster County had just 710 residents, while Buchel and Foley counties had only 298 and 25 residents respectively. By 1897 Buchel and Foley counties had still not been organized, and in that year their territory was officially added to that of Brewster County, making the latter the largest county in Texas.
Jeff Davis County
The prosperity that accompanied the return of the troops, after the Civil War, led to the organization of Presidio County, which had been formed out of Bexar Land District in 1850. Two previous attempts to organize it had failed. Finally, on May 12, 1871, the county was organized, with Fort Davis as the county seat. In 1882, however, when the Southern Pacific built through the area, it bypassed Fort Davis. The residents of Marfa thought that their town, which was on the railroad, should be the county seat. A new election was held on July 14, 1885, and although the results were disputed, Marfa won. The residents of Fort Davis immediately called for a new county, and on March 15, 1887, an act of the state legislature established Jeff Davis County. "Thank God," said one legislator, "that at last we have a Texas county named in honor of the president of the Confederacy." Fort Davis was again a county seat, but the rivalry with Marfa continued as a dispute about the county line. The boundary quarrel, eventually decided in the courts, was not settled until January 1905, when Jesse W. Merrill and S. A. Thompson surveyed a new county line.
One of the primary tools for genealogical research is Federal Census information. These records provide vital statistics and leads as a researcher progresses backward in time seeking ancestors. The first U S Census was conducted in the year of 1790. The practice has continued every ten years since that time. The information contained in each census varies somewhat. At the present time, the US Census for the years of 1790 through 1920 are now public information and available at many research centers and for purchase and rent.
The parent county for Brewster, Jeff Davis, and Presidio Counties was the Bexar Land District. It was organized in 1836 from an old Mexican Municipality.
Due to the remoteness and harsh environment as well as raids and attacks by hostile Indians, Brewster, Jeff Davis, and Presidio Counties did not begin to see any significant permanent settlers and growth until the 1870-1880's.
The official organization of the counties were as follows: Presidio County 1850 from Bexar; Jeff Davis 1887 from Presidio; Brewster 1887 from Presidio.
A researcher would need to check the appropriate county in the time frame in which they are seeking their ancestor.
The following explains the specific information recorded on the census' and will be of assistance to researchers.
Name; address, age, sex, color, whether deaf or dumb or blind; insane or idiotic, all free persons required to give value of real estate owned; profession, occupation or trade of head of each male person listed, place of birth; whether married within the year; whether attended school within the year; whether unable to read and write for persons over 20, whether a pauper or a convict. Officially, the 1850 census lists Presidio County as NP (No Population).
Name; address; age; sex, color, whether deaf, dumb, blind, insane or idiotic; all free persons required to give value of real estate and of personal estate owned; profession, occupation, or trade for each male and female over 16; place of birth (state and territory or country), whether married within the year; whether attended school within the year; whether unable to read and write for persons over 20; whether a pauper or convict
Name, address, age, sex, color, citizenship for males over 21, profession, occupation or trade; value of real estate, value of personal estate, place of birth, whether father or mother were foreign born, born within the year, married within the year, attended school within the year, whether able to read and write for persons 16 year old or over, whether deaf, dumb, blind, insane or idiotic.
Name, address, relationship to head of family, sex, race, age, martial status, born within the year, married within the year, profession, occupation or trade, number of months unemployed during census year, whether a person is sick or temporarily disabled so to be unable to attend to ordinary business or duties-if so, what is the sickness or disability, whether, blind, deaf, dumb, insane idiotic, maimed, crippled or bedridden, attended school within the year ability to read and write, place of birth of person, place of birth of father and mother.
This census was conducted as required by law. However, a major fire occurred in the Commerce Building in Washington, D. C. after the work. All of the 1890 Census burned except for a very few states which have scattered scraps available.
Name, address, relationship of each person to head of family, color or race, sex, date of birth (month, year) age at last birthday, whether single, married widowed or divorced, number of years married, mother of how many children, number of those children living, place of birth of person, place of birth of father and mother, citizenship, year of immigration to US, number of years in UD, naturalization, occupation, trade or profession, ability to read, write, speak English, ownership of home, owned or rented, is home owned by head of member of family, is home free from mortgage, if head of family is a farmer-is farm owned or rented head of house or member of family, if owned is farm free from mortgage, if mortgaged, give number of farm schedule.
Name, address, relationship of each person to head of household, sex, color or race, age at last birthday, single, married, or divorced, number of years of present marriage, mother of how many children, number of thee children living, place of birth of each person, place of birth of father and mother of each person, occupation profession of person, citizenship, year of immigration to US, naturalized or alien, whether survivor of Union or Confederate Service-Army, Navy, Marine.