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In today’s music, there are certain musical features that makes music jazzy and good and I think those are the use of 7th chords and tensions. And of course there are Slash chords(along with them, if sus4 chords when containing 9,4,7 are considered a slash chord too. This sounds so awesome.)

Slash chords has some kind of effect that makes them as equally good as 7th or chords with tension. Could you explain as to what makes it so?

I once try to find 7chords but in Mozart of Beethhoven, they only use diminished and dominant 7th chords, it seems, except on the sonata piece where Mozart daringly uses m7-7-M7-M7-m7-7 in his KV332 sonata. But they from time to time uses those slash chords that has a strong effect. I read once that jazz musicians invented slash chords but that must mean something different to the chords used in past music?

And vaguely when music stared to use 7 chords purely for thier effect? I can only name Gymnopedie so far.

So, the questions are... Q1. What makes slash chords equally sound good as 7th and the ones with tensions in contemporary musics? I’m thinking of contemporary music as those of jazzy ones, r&b, funk and maybe in Steely Dan. I’m sorry I can’t name specifically what chords since there are many but if you want I’ll get an examples of them.

Q2. I’ve read that Jazz musicians kind of invented or modified slash chords to their tastes. To what extent are these chords modified? Simply by adding 7th and using tension notes?

Q3. Since when 7th chords was recognised of its value in music history? It seems to me that 18th century’s music was very careful in using them but didn’t fail to know what these chords can do. Was it so dissonant and unfamiliar in their ears?

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7th, 9th, 11th, and 13th chords were certainly not "invented" by jazz musicians. They are all described in the standard 19th-century harmony textbooks - for example Prout https://imslp.org/wiki/Special:ImagefromIndex/240170/torat (Chapter 13 onwards), MacPherson https://archive.org/details/melodyharmonytre00macpuoft (Part III), etc.

Many of them were decribed even earlier, for example in CPE Bach's "Essay on the true art of of playing keyboard instruments" first published in 1753, but since that book was not influenced by Rameau's publication (1744) of his theoretical discovery of chord inversions, and doesn't make use of the principle that "chords" can be constructed by stacking intervals of a third, it isn't a simple task to re-order CPE Bach's discussion of these chords into the same logical sequence (and the same naming conventions) as a modern harmony textbook.

"Slash chords" have been used for at least 1,000 years in western music, at least over long sustained bass notes. Even "polychords" (e.g. a G7 chord over a C major chord) occur in Beethoven, e.g. the piano sonata Op 81a "Les Adieux", near the end of the first movement.

Whether jazz musicians rediscovered them independently is a different question, of course (and not one that I can answer).

In fact, the Baroque period was generally more "adventurous" with 7th chords of all types than the early Classical composers. J S Bach certainly used them for "effect" - for example the string of diminished 7ths towards the end of BWV715:

. (There are better performances of this on YouTube, but this one shows the score).

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