It is the button on the end of the spindle that is used for adjusting the time and/or date on an analogue (non digital) watch. In older watches and pocket watches which are not powered by batteries it is also used to wind the watch spring to give it power. Generally the winding is done with the crown flush against the watch, pulled out once it adjusts the date (if it has one) and pulled out a second "click" it adjusts the time. The crown is generally round and ridged to provide grip on the finger.
To change the watch crown, the back of the watch must be taken off. The inner workings of the watch are then removed, and the old crown taken out. The process is reversed to put the watch back together. Some of the fancier watches have a crown that can be unscrewed from the outside.
A crown stopper is a feature found on some watches that prevents the crown from being pulled out inadvertently, thus protecting the watch from water damage. It can be a screw-down crown or a push-in crown with a locking mechanism to secure it in place.
crown: Also called a stem or pin, a crown is the knob/button on the outside of the watch case that is used to set the time and date. In a mechanical watch the crown also winds the mainspring. In this case it is also called a "winding stem". A screw in (or screw down) crown is used to make a watch more water resistant. The crown actually screws into the case, dramatically increasing the water-tightness of the watch.
Rolex
I own an AT0875-09E, and can report that the crown does not screw down, although the watch is WR200. I have swum with it with no problems.
To set the time on your TechnoMarine watch, first locate the watch crown on the side of the watch. Pull the crown out to the appropriate position (usually position 2 for time setting) and then turn the crown to adjust the hour and minute hands to the correct time. Once you have set the time, push the crown back in to secure the settings.
To set the time on a Charles Raymond watch, locate the crown on the side of the watch. Pull the crown out to its second position and rotate it to adjust the time. Once the correct time is set, push the crown back in to its original position.
To set the date on a Timex Indiglo watch: Pull the crown out to the first position. Turn the crown clockwise to advance the date. Once the date is set, push the crown back in.
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To set the time on a Michele watch, find the crown on the side of the watch and gently pull it out to the first position. Rotate the crown until the correct time is displayed, then push the crown back in to secure the time setting. Ensure the watch is wound and running before adjusting the time.
To wind a Baume et Mercier automatic watch, hold the watch in one hand and unscrew the crown located on the side of the watch. Gently pull the crown out to the first position and turn it clockwise for about 30-40 turns. Push the crown back in and screw it back tightly. Your watch should now be wound and ready to wear.
To set the time on a Stauer watch, pull the crown out to the time-setting position. Turn the crown clockwise or counterclockwise to adjust the time to the desired hour and minute. Push the crown back in once the correct time is set.