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Commentary
According to all his immigration, naturalization, marriage and census records, Gerhard was born in either 1864 or 1865. His father, Joannes Mayer (b. 1829), was married twice, first in 1856 to Catharina Reichert (no record of her death has been found) and second in 1859 to Anna Ross. We know that these were the wives of the same man because both marriage records list identical parents for Joannes. The last record for Catharina Reicharts while living was in 1857 when her first son Joannes (later John) was born. This John Mayer immigrated to the United States in 1883, and listed Catharina Reichert as his mother at the time of his marriage. When Gerhard immigrated at the age of 28, he listed his brother John as his contact. Catharina Reichert had at least one other son who died in 1862, but he must have been born before the father remarried (i.e. before 1859).

Joannes's 2nd wife Anna Ross had 7 children between 1860 and 1870 for whom birth records have been found, all of whom died very young. When Anna died in 1873 at the age of 41, her step-son John was 16 years old and Gerhard was 8 or 9. There is no record of their father's death, but certainly they grew up in a household filled with tragedy as 10 out of 13 family members died in the span of about 13 years.

Unfortunately, no record of Gerhard's birth was found along with the other children, so we cannot be certain who his mother was. If he was born in 1864/5 as all his original records indicate, then Anna Ross must be his mother as Catharina was already deceased. Gerhard, however, listed his mother as Catharina Reichert in his 1896 marriage record. Why would Gerhard list Catharina as his mother instead of Anna? There are several possible reasons; it might be that he did not remember his mother's name as she died when he was young and his brother John, whose mother was Catharina, may have provided him with incorrect information, or perhaps more likely, he was actually born earlier than 1859 and forgot his actual birth year by the time he reached his 20s, which was not as uncommon as one might expect. Neither explaination is very satisfying, but unless a German DNA connection if found to the ancestors of one of these two women, it is doubtful if we will ever no for certain who Gerhard Mayer's mother was.
Family
ClaimDetailEvidence
FatherJoannes Mayer (1829-) [S2169:35]
primary
SpouseMargaretha Dahm (1872-1958) 
Child +Clara Elizabeth Mayer (1900-1983) [S4] [S258] [S486] [S1794] [S2890:19490209]
dna
Attributes
ClaimDetailEvidence
GenderMale
NameGerhard Mayer [S14] [S21] [S26] [S258] [S815] [S1505] [S1790] [S2182] [S2880] [S2890:19490209] [S2891] [N126]
primary
NameGerhard Maier [S1769]
primary
NameGerhart Meyer [S1794]
primary
NameGerhardum Mayer [S2169:35]
written in Latin
primary
NameGerardus Mayer [S2170:78]
written in Latin
primary
FatherJoannes Mayer (1829-) [S2169:35]
primary
Possible motherAnna Ross (~1832-1873) [S2693]
secondary
Probable motherCatharina Reichert (1834-) [S2169:35]
primary
OccupationGrocer [S2182]
primary
Ship [S1769]
primary
NaturalizationPennsylvania, United States [S1794]
primary
Timeline
ClaimDateDetailAgeEvidence
Birth1864Germany [S258] [S1769] [S2891]
primary
BirthJun 1, 1864Wetteldorf, Prüm, Rheinland, Trier, Rhineland-Palatinate, Germany [S815] [S1505] [S1769] [S2169:35]
primary
Birth1865Germany [S428] [S486] [S1790] [S1794] [S2169:35] [S2170:78]
primary
BirthJun 1, 1865Wetteldorf, Prüm, Rheinland, Trier, Rhineland-Palatinate, Germany [S2169:35] [S2170:78]
The first instance we have of him indicating his date of birth is in the 1896 marriage register where he stated that he was born in June 1, 1865. By the time of his 1914 naturalization record, he was using June 1, 1864. This is the year his children and grandchildren thought was correct. His eldest son's baptism record proves he was born in Wetteldorf, Germany, not Vittledorf as his death certificate states, or Nettledorf as his brother's death certificate states, or Schoenecker as my mother had written down (locations that do not exist in any geocoordinate databases).
primary
Emigration1892Germany [S258] [S428] [S1769]
primary
ImmigrationMay 11, 1892New York City, New York, New York, United States [S258] [S428] [S486] [S1505] [S1769] 28y
primary
Immigration1893 [S1794]
primary
Emigration1893Germany [S1794]
primary
MarriageFeb 12, 1896Margaretha Dahm (1872-1958), Cook, Illinois, United States [S14] [S26] [S258] [S428] [S1794] [S2169:35]
primary
Occupation1900Molder of Iron [S1794]
primary
Census (US Federal)Jun 12, 1900531 West Hasting St., Westown, Chicago, Cook, Illinois, United States [S1794] 35y (Named)
primary
Residence1905901 School, Chicago, Cook, Illinois, United States [S2182]
primary
Residence Jun 20, 1907 to 19093937 N. Clark St., Chicago, Cook, Illinois, United States [S2182] [S2880]
primary
Occupation1910Grocery Store Prospector [S258]
primary
Census (US Federal)Apr 15, 19105706 N. Clark St., Chicago, Cook, Illinois, United States [S258] 46y (Named)
primary
Residence 1911 to 19165706 N. Clark St., Chicago, Cook, Illinois, United States [S2182]
primary
FlourishedJan 8, 1914Cook, Illinois, United States [S1505]
denied naturalization
secondary
NaturalizationSep 19, 19145706 N. Clark St., Chicago, Cook, Illinois, United States [S1505]
secondary
Census (US Federal)Jan 1, 19206311 Wagner Ave., Chicago, Cook, Illinois, United States [S486] 55y (Named)
primary
Census (US Federal)Apr 1, 19301456 Edgewater Ave, Chicago, Cook, Illinois, United States [S428] 65y (Named)
primary
ResidenceApr 1, 19351456 Edgewater Ave, Chicago, Cook, Illinois, United States [S1790]
primary
Census (US Federal)Apr 9, 19401456 Edgewater Ave, Chicago, Cook, Illinois, United States [S1790] 75y (Named)
primary
OccupationFeb 7, 1949Retail Grocer [S815]
primary
DeathFeb 8, 19491456 Edgewater Ave, Chicago, Cook, Illinois, United States [S21] [S815] [S2890:19490209] [S2891]
Colon Cancer
84y8m7d
primary
ObituaryFeb 9, 1949Chicago, Cook, Illinois, United States [S2890:19490209]
memoir
BurialFeb 11, 1949St. Boniface Cemetery, Chicago, Cook, Illinois, United States [S815] [S2891]
primary
Sources
IndexTitle
[S14] "Illinois Statewide Marriage Index 1763 - 1900" (October 25, 2005) (https://www.ilsos.gov/departments/archives/databases/marriage.html).
[S21] "Illinois Statewide Death Index 1916-1972" (October 25, 2005) (https://www.ilsos.gov/departments/archives/databases/idphdeathindex.html).
[S26] "Gerhard Mayer and Margaretha Dahm Marriage Certificate".
[S258] "1910 United States Census, Illinois, Cook, Chicago, Ward 26".
[S428] "1930 United States Census, Illinois, Cook, Chicago, Ward 49".
[S486] "1920 United States Census, Illinois, Cook, Chicago, Ward 25".
[S815] "Certificate of Death: Gerhard Mayer".
[S1505] "Gerhard Mayer Naturalization Record".
[S1769] "New York Passenger Lists".
[S1790] "1940 United States Census, Illinois, Cook, Chicago, Ward 49".
[S1794] "1900 United States Census, Illinois, Cook, Chicago, West Town".
[S2169] "St. Peter's Marriage Register".
[S2170] "St. Alphonso's Baptism Register".
[S2182] "Chicago City Directories".
[S2693] "Germany, Rhineland, Diocese of Trier, Catholic Church Records, 1704-1957", Family Search (https://www.familysearch.org/search/collection/2790268).
[S2880] "Newspaper: The Inter Ocean (Chicago)".
[S2890] "Newspaper: Chicago Tribune".
[S2891] "St. Boniface Cemetery Tombstone Photographs" (http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=60538705&ref=acom).
[N126] Tim Forsythe, "A Gerhard Mayer Discovery", April 3, 2015, online article , Papa Greybeard, Greybeard's Ghosts, (https://timforsythe.com/greybeard/blog/a-gerhard-mayer-discovery, accessed: December 5, 2023).
Family Note
Gerhard Mayer was the son of Johannis Mayer and Catherine Reicherts. He was born on June 1, 1864, in Germany. He died on February 8, 1949, in Chicago, Cook County, Illinois. Gerhard immigrated to the United States from Germany in May of 1892 . He may have been from Vittledorf, Germany (S. Bolivia).
According to her certificate of baptism dated June 2, 1872 and occuring in St. Peter's Catholic Church, in Niles Center, Cook County, Illinois, Margaretha Dahm was the daughter of Johannis Dahm and Catharina Wagner and was born May 21, 1872. Margaret was confirmed on 11 June, 1888. She died on February 18, 1958, in Chicago, Cook County, Illinois. She was called Maggie.
Gerhard and Maggie were married on February 12, 1896, in Cook County, Illinois, presumedly in Chicago.

Gerhard and Maggie had 5 children:
Catharine Josephine (1896 - 1896)
Susan Angela (23 Feb 1898 - 1 Jun 1932) m. John "Jack" Hensler (Children: Mary, John A., Robert John, Susan (m. Bob Busch))
Clara Elizabeth (3 Jun 1900 - 26 Apr 1983) m. Fredrick Albert Loebach
John A. (~1903 - ) m. Viola Greiner (Children: Johnny b. ~1936 and twin boys)
Ernest Gerhard/A. "Ernie" (13 Jun 1906 - 11 May 1975) m. 23 Jun 1934, Katherine L. "Kay" O'Rourke (Children: John Robert "Jackie" b. ~1934 and Nancy Kay b. 18 Jan 1938)

Ernie was undoubtedly named after Ernst Simon who married Maggie's oldest sister and was a witness on Gerhard's naturalization.

Gerhard owned and operated a grocery store in Chicago, Illinois. Maggie was a seamstress. They owned a 6 flat on Edgewater Avenue. Their daughter Clara and family lived in the apartment above them. The other 4 apartments were rented out.

In 1914 a John Mayer is listed as a witness on Gerhard Mayer's naturalization record. This John Mayer was living seperately, so could not be his son who would have been at the most 16 years old. This John Mayer may have been a brother. He was born in 1858 and immigrated in 1883. Gerhard named his first son John as well. Probably this older John Mayer is an oldest son, and their father was named John.
Family Note
[URL: https://vitacollections.ca/IllinoisNews/details.asp?ID=3469609&n=12]
Notes for Ernest A. Mayer
Spouse: Katherine
Survived by: Three sons, John R. Mayer of Chicago, Gerard F. Mayer of Miami, FL, and James M. Mayer of Boston, MA; daughter, Nancy Kay Mayer of Evanston; sister, Clara Loebach of Chicago; and two grandchildren.
Age 68.
Cemetery: All Saints Cemetery, Des Plaines
Date of Publication : 15 May 1975
Date Of Event : May 11, 1975

https://vitacollections.ca/IllinoisNews/details.asp?ID=1660565&n=3
Katherine O'Rourke Mayer, 90, of Evanston, formerly of Wilmette, born in Minonk, Illinois, died November 21, 1998. Survivors include one daughter, Nancy Kay Mayer of Evanston; three sons, John R. , James M. (Kristine) and Gerard F. (Betsy) of North Barrington; one grandchild; and one sister, Mary Seguin. Preceded in death by her husband, Ernest A. Mayer
Date of Publication
26 Nov 1998

PeopleFinder, etc. -
Nancy Kay Mayer 83 (2020) Evanston, IL
Relatives:
Gerard F Mayer (? His birth date was listed as 01.23.48)
James Michael Mayer (? He was born on January 22, 1948. lives Barrington, IL)
Katherine O Mayer