The connections on the headphone plug are labeled, "Tip" which is the tip, "Ring" which is one of the flat silver rings, and the "black lines" are the insulators between the connections, and the longer back end silver portion is labeled "Sleeve". The Tip is the Left channel (and can also be the Mic input), the first Ring is the right channel (and can also be a mic input), the second Ring (if one exists) is either the Mic input by itself or a selector input such as for a button, and the Sleeve is Ground for all connections. . The plugs that have 2 black insulators are plugs with 3 electrical connections. The plugs with 3 insulators are 4 connection plugs. There are also 4 connector plugs though far less common.
Using a 4 connector plug in a 3 connection socket can result in one of the connections not being used or being shorted to ground (depending on how the plug is configured internally. This may prevent the headphones from performing all intended functions. Fortunately for you, the cord extender you had must have a slightly different configuration of the connectors such that it was able to make connection with the necessary ring to allow full functionality of your headphones. Not all cord extenders will have the same results, so it's best to stick with a pair of headphones that has the proper number of connections to match the phone's input jack.