To add to the information from the other answers to this question:
Years ago, prior to the extensive availability of ATC radar, it was not uncommon to use Alternate Airways for IFR enroute lateral separation between aircraft .
In order to achieve and maintain separation between aircraft in a non-radar environment ATC procedures could be complex and inefficient (compared to the use of radar separation). One procedure that reduced the complexity was the use of Alternate Airways. These airways were located in areas where, based on the traffic flow, were beneficial for both ATC and pilots.
For example, consider if one aircraft is at a higher altitude than a second aircraft and they are in close proximity and assigned the same (Victor Airway) route. If the aircraft at the higher altitude needs to descend to an altitude below the second aircraft some form of non-radar separation (longitudinal or lateral) needs to be achieved prior to allowing the aircraft at the higher altitude to descend. One method to achieve this as efficiently as possible was the development and use of alternate airways. (see the sample image below for an example)
If the angle between the primary Airway and its alternate was 15 degrees the point at which ATC lateral separation would exist would be 16 NM from the Navaid. If the angle between the primary Airway and its alternate was greater than 15 degrees the point at which lateral separation would exist would be less than 16 NM. (see FAA Order JO 7110.65Z, Section 5, Lateral Separation Non-Radar)
The image below is just one example of the use of Alternate Airways, which likely no longer exist anywhere in the U.S. due to the availability of radar throughout most of the enroute structure.
Lastly, the foundational design of the IFR ATC system was (and is) based on Non-Radar routings and loss of communication between ATC and pilots (with some exceptions). The elimination of Alternate Airways is a product of the now expansive availability of ATC radar coverage resulting in the reduced need for using Non-Radar separation (longitudinal and Lateral) procedures. (Vertical separation is Non-Radar separation even when used in a radar environment)