A bar = one measure in a 4/4 rhythm... whatever the tempo may be usually 88-94 beats per second so it's tap tap tap TAP one bar. Most bars are anywhere from 8-14 syllables 14 pushing it unless you're twister. You can also look at it like the kick is always on the 1 than a snare on a 2 and another snare on the 4. The kick signaling the begininng of the bar, and the snare on the end of it. You usually want to make sure your drop is synced with the kick, and the snare with the last rhymed word. It's also a good idea to sycnh your syllables to perfect your multi flow in couplet bars. Each line I write fits in a 4/4 bar and can be spit on beat to = 16 bars.
Further more, the structure of most beat production, is an intro/drop, 16 bar verse, 8 bar hook, than another 16, this is the typical sequence of a beat which is why 16's are common.
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