There is a chart that gives seat dimensions and the location of movable armrests: https://www.southwest.com/help/on-the-plane/our-airplanes; Southwest does not assign seats but customers who need a specific seat may pre-board
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Adding assistance when booking: On desktop, go to the Special Assistance link on the "Passenger & Payment Info" screen. On mobile: Go to the Special Assistance link on the "Passenger" screen. Adding assistance to an existing booking: On desktop, manage your reservation and select "Special Assistance" under the passenger name. On mobile, manage your reservation and select the edit pencil icon next to the passenger name. Then select "Special Assistance". You can also let us know via telephone by calling 1-800-I-FLY-SWA (1-800-435-9792).