In the Indiana State Highway numbering system, one and two digit numbers are considered “major” state roads. When the Great Renumbering occurred on 1 October 1926, there were many numbers that were left out, only because it would leave the State Highway Commission room to add to the system without ruining the numbering scheme. One such road started by connecting Peru to Marion, but would ultimately be extended to Richmond. Only to be removed again in the end.
SR 21, as mentioned, connected the county seats of Miami and Grant Counties. The first reference to this road starts on the early 1929 Indiana Official Map. At the time, the road was routed from Peru through Santa Fe, North Grove, Amboy, and ending, officially, Converse, with an authorized addition through Mier and Sweetser ending in Marion.
Later that year, with the release of the late 1929 official map, the route had been completed to Marion. But there was another section that left Muncie to the northwest, ending at SR 28. Both the section at Muncie and from Converse to Marion would be removed again for the early 1930 official map. And appeared again on the late 1930 map. That late 1930 map would also show a “dotted line” road, or authorized addition, connecting the end of the Muncie section to Marion, and from Muncie to SR 38 northwest of Richmond.
At this point, SR 21, which originally was between SR 15 and US 31, fitting into the numbering scheme, the additions from Marion to the southeast would cross several odd numbered routes.
By 1932, the above mentioned authorized additions were officially recognized. The Muncie to Marion section would connect through Stockport, Wheeling, Matthews, Jonesboro and Gas City. From Muncie, the road connected to northwest of Richmond via New Burlington, Blountsville, Losantville, Economy, Williamsburg and Webster. Another authorized addition, this time in 1932, would soon became a daughter to SR 21, to be numbered SR 221. This would leave SR 21 at Matthews, heading due north.
It should be noted that another road that looks like a daughter of SR 21, SR 121 northeast of Richmond, is not. SR 121 is a continuation of SR 121 in Ohio. The fact that SR 21 got close to Richmond made the number fit into the system semi-nicely.
Other than upgrading the road to hard surface over several years, no real changes to SR 21 had occurred until 1935, when it was multiplexed with US 35. At least to Gas City. While kinda. On the 1935 map, the SR 21 designation only appeared on the Gas City to Peru section. That would change in 1936 when US 35 and SR 21 were sharing the same lines on the map.
The next change to SR 21 would occur in 1950/1951 when the designation from SR 18 west of Converse to Richmond was removed. The section from Marion to Gas City was renumbered SR 15. The rest of the road’s number was removed, since it was a multiplex with either US 35 or SR 18.
Even that would change in 1968, when US 35 was rerouted along SR 28 to meet with I-69, to share that road between SR 28 and SR 21. This left SR 221 an orphan from Matthews to SR 26. It would stay that way until 1972, when SR 221 was removed except between SR 18 and SR 218. And completely removed in 1975.
The last section of SR 21 was removed from Indiana maps in 1980, when the number of the road was changed from SR 21 to SR 19 between Peru and SR 18 south of Amboy. Thus, after 60 years of existence, SR 21 joined SR 34 as a decommissioned major state road number.