J channel siding is used to finish the ends of vinyl siding, around doors, windows, and on corners. It gets its name from the J shape it has when looking down from the top of the piece.
J channel siding will need to be measured accurately to allow for proper installation. The measurement is taken from around the door or window, or on the corner it is being installed. Measure from the outside of the existing trim for the best fit. The channel needs to be given room for the siding to expand and contract with varying temperatures. The siding should be cut slightly shorter in the areas where the J channel will be installed. Be sure that you aren't cutting it so short that the siding will fall out of the channel.
If you cut the ends at a 45-degree angle, and create a small cut on the top piece of the J channel, water will be able to run down the channel without getting caught inside. This will prevent water damage that can occur when water is allowed to get underneath the siding itself. Many manufacturers will include a small perforation along the channel that isn't easily seen just for this purpose.