Don't you just show the notes ? Although they can be very hard to read.
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I guess that would depend on the style of music, and your intended result.
If it's important that you can determine the exact notes that get played, then I would write it just like a chord - arranging all the notes around the stem, and making the accidentals clear, can be a little awkward. It's a long, long time since I studied scores, but I have a feeling that some of Ligeti and Pendereckis' scores have examples of this type.
On the other hand, if you simply want the effect of a very dense set of notes, and you're not too concerned by the precise content, I've seen some composers notated a top and bottom note with a thick black line joining them. I think the Concord Sonata has clusters notated in this way, but I can't really remember.
+1 for Pingu's suggestion that these kind of things, clusters, can be indicated rather than literally notated. If you studiously wrote it all out - "OK now, I want a B, and a C, and a C#, and a D, and an E, and an F"--would that cacophonous cluster sound all that much different to you than if a group of musicians ad-libbed a cluster based on a notation of "B squiggle mark to F"--each musician grabbing one of the notes in the series at random? It would make No Difference.
And for composing a piece that needed things like that, I advocate staff paper taped to a dart board, and then you throw the darts-- Or you assign notes to cards in a deck, and you randomly throw a bunch into a hat, and then-- Or you assign notes to the colors of the spectrum, then shine a light through a prism, and--
---and so on. 8-)
According to wiki (I found it via the German 'toncluster') it's a vertical line.
Try here: http://en.wikipedia.org/wiki/Tone_cluster.
The vertical squiggle is for an arpeggio and is often found in harp music.
I come across it quite a bit in Puccini's scores.
I think it somewhat depends on how they are used, at what tempo and note duration, and a little on what genre. All the shortcuts seem valid (especially Randy's prism method ).
If you just need a <BLAHT> of sound or color (or Fx), fine for the shortcut, especially if it's all on a piano or keyboard.
But to me, unless the cluster is all m2's or all M2's or all +2's, the shortcuts can't mix and match different 2nds clearly. If it's an orchestrated cluster, the composer/arranger may chose certain notes in the cluster based on the voice-leading of the parts into and out of the chord. If the cluster is at slow tempo or is long-duration notes, unless you have all 12 tones in the cluster, it will sound different and should be written out. I'm most experienced w/ jazz scores, and unless it is purposely a group improvisational piece, who plays what note in the cluster is generally written out.
All answers worth studying. Thanks.