On many occasions, I have attended events where I was impressed by a DJs ability to blend songs into each other (also known as beatmatching and mixing). As a matter of fact this practice is part of the fundamentals of DJing.

Indeed a DJ should be able to seamlessly make a transition from one song to anothe,r by picking songs that have similar tempi, and adjusting the speed control to align the pieces so that they become one layered rhythm.

Traditionally, this was only possible when a DJ was using vinyl but when technology made it possible to use digital media to mix (e.g. CDs, laptops) it became even easier to calculate the tempo of a song and match it to the current one playing.

In a way, beatmatching is a technical exercise that does not necessarily involve understanding music theory but it requires taste. I have personally attended an event where a DJ played songs at the same tempo for over an hour. While that is not something unusual, it is important to understand that in this situation, a lot of the pieces were sped up to the point that some of the vocals sounded like high-speed dubbing!

Mister/Madam DJ, you have to understand that you carry a responsibility to ensure that people who are listening to your sets enjoy them and don't develop ear fatigue. There is a reason why some songs are slow and others are fast. Also, if you are mixing one song into another and chords are involved, you need to take into account chords and how they go or clash with each other.

On that note, while technology has made DJing more accessible, it has also created new potential problems. Next time you go to an event where a DJ is playing, do not be surprised if you hear your favourite song sped up without the pitch changing.