I don’t know why musician’s use the start of the count typically, I am not a drummer but if I ever counted off for my band I usually used the end of the count because, and I assume this is the same for dancers, it tells you how long the phrases are, and we didn’t always play in 4/4 or off 4 counts. Most rock music is played in sets of 4 (counting 1234 1234 1234 or 1+2+3+4 or 1+a2+a etc), most dances are choreographed in counts of 8. But they don’t have to be done that way. Some dances will be phrased in counts of 4 (unusual I know, but I have seen it). Some will be phrased in counts of 6, some in counts of 3. Counting to the end of the count lets you know how the movements are grouped right off the top, and as musicians will tell you (and I assume dancers too) the way you count changes the way the music feels.
Where the slowed down 5 6 count comes from, I have no idea. I haven’t actually seen anyone use it in real life, though I believe that some people do. Maybe it’s a way to set their own timing? Maybe Fosse did it and everyone else is now imitating him? Who knows. I am also not a choreographer, but in my time in the theater I have never seen it used. I usually see the choreographer just hold up a hand and shout something like “ready” then count into the 5678.
This phrase was coined by the famous choreographer and dancer Luigi with help from his friend Dominic Frontiere. Dominic would say “1-2-3-4” and Luigi would say “ah 5-6-7-8!” The phrase was then spread throughout the dance world due to the fact that Luigi was so famous… The phrase is now a common one in a dancers vocabulary.
Source:IN PERSON; 'Never Stop Moving' - The New York Times
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I had a squad mate who, when it was his turn to lead the squad in exercises (jumping jacks and the like) would start us off with “and a 5, 6, 7, 8!” We’d get a quick chuckle out of it. The platoon sergeant was less amused and the LT would struggle a bit to maintain his Military Bearing*
I know a bit of music but nothing about dance. 4 4 is a time signature that musicians find useful and common. I see no reason why what works for musicians will necessarily work as well for dancers so if they like a dance measure that’s equal to 2 music measures, that’s fine with me.
I’ll note that musicians might just count 3, 4 and then jump in on the one. Not musicians like me, mind you, but real musicians who are ready and confident. In fact, I’ve heard that good musicians who know the tempo can count 4 and have the band start playing on one. Never seen it with my eyes but heard about it from an old teacher. Naturally dancers want to save time and get to work so shortening to 5 6 7 8 just makes sense.
Jazz musicians often set their metronomes at half the tempo they want so they click on 2 and 4. It mimics the high hat in a lot of jazz. They listen to the metronome long enough to get the tempo and then count 1 pause 2 pause 1 2 3 4. The first 1 2 come in the spaces between clicks and by then they’re ready for a 1 2 3 4 where the 2 and 4 are timed with the clicks on the metronome. iamthewalrus(:3= is absolutely right that, if musicians were logical, they’d count 1 3 1 2 3 4 but, ya know, tradition. Seems reasonable that dancers, who are no more logical than musicians, would count 5 6 5 6 7 8 after adapting it to their eight count thing.