2010 ADA Standards for Accessible Design Pocket Guide
Car control buttons with floor designations shall comply with 407.4.6.2 and shall be raised or flush.
EXCEPTION: In existing elevators, buttons shall be permitted to be recessed.
Buttons shall be 3/4 inch (19 mm) minimum in their smallest dimension.
Buttons shall be arranged with numbers in ascending order. When two or more columns of buttons are provided they shall read from left to right.