 In view of the frame-dependence of simultaneity for events at different locations in space, some treatments prefer a more phenomenological approach, describing what the twins would observe if each sent out a series of regular radio pulses, equally spaced in time according to the emitter's clock.This is equivalent to asking, if each twin sent a video feed of themselves to each other, what do they see in their screens?Or, if each twin always carried a clock indicating his age, what time would each see in the image of their distant twin and his clock?