The quickest way to connect two ropes is to tie them together. The most secure way to connect two ropes is to splice them together. When you need to connect two lines you will have to weigh out the pros and cons of each option to decide what is best for you at that moment
If you have plenty of time and the skill needed, splices will offer the best connection. If you are in a rush, a simple knot might meet your immediate needs.
Splices offer a secure, permanent connection which results in no loss of strength. This is because a properly executed splice will evenly distribute the forces across all of the fibers in the line without creating pressure spots of increased load and tension. The disadvantages of splicing is they can be time consuming and require a skill set for each type of line.
Lines are characterized as Three strand, Single Braid, and Double Braid. Within the single and double braid, they are then broken down into Class I and Class II fibers. Each type of line (three strand, Class I, and Class II) has its own rules about splicing. As you can see, it can get complicated in a hurry!
The next option is to simply tie the lines together by way of a knot. Knots create areas of increased stress which weaken the line. The bowline is known as the knot that retains the most strength in the line; retaining approximately 80% of the lines strength. Other knots will be more detrimental to the lines total strength.
Some knots that can be used to securely connect two lines are the "sheet bends" and the "fishermans knots". These two knots will securely hold the two ropes together without much risk of the lines separating. They also work on lines that are of different sizes.
You can connect lines with square knots, but these knots are not as secure as a sheet bend or fisherman knot.
If the loss in strength is no acceptable and you do not feel confident in your ability to splice the lines, you might be wondering how else you can connect these two lines. Another option that does not involve splicing or tying the lines together is to seize the lines together.
If you properly seize the lines to one another, they will not slip nor yield. They will be locked together without the time of splicing or the stress of knotting. Seizing will require an additional piece of line, and can act as a permanent solution.
For joining two lines together, it would be best to use a "Flat Seizing Knot". This knot is used to bind two lines of similar size together and never let go. Tying three of these knots in succession will provide a permanent junction that will hold the lines together and evenly distribute the forces over the length of the seizes, providing you with piece of mind.
Tying a Flat Seizing Knot is a lengthy ordeal, taking one much longer than it would to tie a Double Sheet Bend. I personally prefer to tie lines together with Sheet Bends, as it is a quick knot to tie and holds securely once tightened. Sheet Bends also allow for easy separation of the two lines once the long line is no longer needed.