There are two ways to input chords, but one will be used much more often.

This style of chord notation is more common: <as ces>4-_ Notice how only the pitches are notated inside the < > brackets, and everything else attached to the end. There is one exception to this: fingering should be indicated on the pitch associated with that finger: <as-1 ces-3>4-_ Not only does this help you to sort out what was probably intended, but it allows LilyPond to stack the fingering in the right order. When using "relative" entry mode, it is the lowest note of the chord that is taken into consideration when placing the next note.

This style of chord notation is less common: <<as4-> ces>> Notice how everything must be notated inside the << >> brackets. This can make it more difficult to read the chord in the source file, but it also allows much greater flexibility: only some chord members can have ties; certain chord members can last for longer than others; certain chord members can "break out" into or out of polyphonic passages. This notation is rarely needed, but you may be inclined to over-use it if you are trying to exactly copy the look of a hand-engraved score. Like the "times" command for tuplets, this is one of LilyPond's deceptively powerful techniques. When using "relative" entry mode, it is the last note of the chord that is taken into consideration when placing the next note.