What is praying the salats?
It is considered a ruhsat to pray the salats that are in the same waqt as one another (such as the zuhr-asr salats and the maghrib-isha salats) by shortening them when making a journey on land, not a sea journey.
There are six reasons for praying the salats by joining them: Being in Arafat, being in Muzdalifah, journey, rain, being surrounded by mud while in the dark, fainting etc.
It is considered jaiz for the traveller, who is found to be on a break after the sun has moved towards the west, to join the zuhr and asr prayers (as in to pray both the prayers in the time of the zuhr prayer) when they have the intention of taking their next break after the sun has set. But if they have the intention of taking their next break before the sun has yellowed, they will have postponed the time of the asr prayer to its normal, voluntary time. If they have intended on giving a break after the sun has yellowed, they may join the asr prayer with the zuhr prayer by the way of the jam at-taqdim, or they may postpone their prayer and join the zuhr prayer with the asr prayer by the way of the jam at-takhir. The first way is better. If the sun has moved towards the west when the person is on their journey, the person who intends on giving a break when the sun yellows or before this time will have postponed the time of their zuhr and asr prayers (they will pray in the jam takhir way.) If they intend to give a break after the sun has set, they will pray each of the prayers in their own time, as in they will pray zuhr in the first waqt and the asr in the second waqt. This is called jam’ al-suri.
The person who does not know when they will give a break and the ill person will also combine (jam) the prayers in this way.
The maghrib and isha prayers can also be joined in this way. In this situation, the rising of the sun will be evaluated like the setting of the sun in the other matter. Also the last two thirds of the night will be considered as the yellowing of the sun. The period before both of these is evaluated as the period before the sun has yellowed.