Canon AE-1 - Manual (Page 41)

shutter release button is turned to the "L"
position, the shutter button will be locked to
prevent unintentional shutter release. Keep
the shutter release button locked while
carrying the camera to prevent film waste.
When the power level of the battery is
insufficient, a safety mechanism will keep the
shutter from being released.
Frame Counter
The frame counter is an additive type
which counts one frame every time the film
advance lever winds the film. When the
camera's back cover is opened, the frame
counter automatically resets itself to the "S"
position.
While rewinding film, the frame counter
counts back the frame numbers. The starting
position "S", 0, and the even numbers 2 to 38
are displayed by the counter. Numbers 20 and
36 are marked in orange to call your attention
to the end of film cartridges such as are today
commercially available. The frame counter
cannot count higher than 38.