# Can I ignore the bottom number in a time signature if I know the BPM? [duplicate]

I never remember what that bottom number means, can I just stick to the top number and call it a day?

So in a 6/8 I could just know that it's 6 beats per measure (ignoring the 8).

If I know my BPM (for example 80 bpm), then does the top number suffice?

• I study music and music theory, but I know plenty of folks who just play and enjoy without doing the study. That's how someone might not know what the bottom number means. Should they be made to feel less than because of it? – skinny peacock Feb 12 '18 at 16:31
• @CarlWitthoft - people should be commended for asking when they are unsure, not mocked. Please refrain from doing that - you can think it, but don't say it. – Doktor Mayhem Feb 13 '18 at 17:23

No. For a start, there aren't six beats in a 6/8 bar. There are two, each beat lasting three quavers. Think 1 2 3 4 5 6. The beats are on 1 and 4. This is called a compound time signature.

The concept of beats per minute is a little hazy because of this. Digital Audio Workstations can make this even more confusing, because they often falsely equate beats per minute with crotchets per minute. That's fine in 4/4, but wrong in many other time signatures.

So to use a BPM marking (or, more generically, a tempo marking), you need to understand what a beat is in a particular time signature. And to do that you need both numbers. It tells you the number and type of notes in the bar, and also the emphasis or grouping of those notes.

Could you muddle along without knowing all this? Probably. But you'll end up with weirdness somewhere down the track (using a DAW in 12/8, for example), and it would be easier to fix if you understand a bit more of what's going on.

• Not entirely true. Sometime 6/8 is six beats measure, but usually only when it's a very slow piece or movement of a larger work. That would be indicated if a tempo marking is given by 8th = 80, instead of dotted quarter = 80, in which case it would be 2 beats per measure. Of course, at any time a conductor could choose to switch back and forth between the two, for example to conduct an extended rallantando and have the ensemble move together cleanly. – Randy Howard Feb 11 '18 at 22:35
• @RandyHoward I understand your point, but you're effectively saying that in that case, 6/8 is not a compound time signature. I'm not sure that's a helpful point to raise, because of the beginner nature of this question. Some would also dispute whether your example is 'correct'. I'm more of a theoretical pragmatic; if the notation efficiently communicates the musical idea, it's fine. But confusing simple and compound meters is probably unhelpful here. – endorph Feb 11 '18 at 23:05
• In music it's done both ways. Whether or not a beginner wants to 'know that' or not is a different question, but along the way, you will eventually encounter it. You even see 4/4 conducted in 8 sometimes, or 2, or even 1 occasionally, depending upon tempo or the desired feel (pulse) that is appropriate. – Randy Howard Feb 11 '18 at 23:24

To be a rounded musician, you should fully understand such things as time signatures. You will easily understand and remember what the bottom number means with a little bit of effort. Take a look at music in question. If it is say 3/4, you will quickly notice that there are (what adds up to) 3 crotchets (or quarters) in the bar. If it is 5/8 you will notice that there are 5 quavers (or eighths) in the bar, etc.

It's the bottom number that tells you what a "beat" is. It could be a half note (minim), quarter note (crotchet) or eighth note (quaver).

For compound time, it's less obvious. In 6/8 a dotted quarter note (dotted crotchet) would be one beat.

Get that wrong, and you could be trying to play the piece twice as fast as you should, or half as fast.

• as mentioned I know the bpm so does that matter – user34288 Feb 11 '18 at 20:11
• You still need to know what note is a beat. In the case of 6/8, it's an eighth note. You could work that out by adding up all the notes in a bar, then working out what you need six of to make up a bar. But the time signature tells you - it's eighth notes. – Simon B Feb 11 '18 at 20:16
• @SimonB -- if the time signature is 6/8 the beat-note is a dotted crotchet (quarter-note). That's why musicians need to invest time in learning how beats & pulses in music work. – Dean Ransevycz Feb 11 '18 at 21:41
• @DeanRansevycz You're quite right. I completely missed that it's compound time. – Simon B Feb 11 '18 at 22:06