I realize that I’m joining a conversation in progress, so excuse me if I repeat what may have been said. It is important to understand where the bowing, genuflecting, tabernacle and altar all came from.
Until the 1200s tabernacles were not on the main altar. The tabernacle on the main altar was an innovation introduced to Roman Catholicism by St. Francis of Assisi. Though it is beleived that others had already started this on a smaller scale. When our Holy Father built our first chapels, they were very small in comparison to the large monastic churches and cathedrals of the time. There was no other place to put the tabernacle but on the main altar.
Let’s understand that Francis wanted the tabernacle present, because of his great devotion to the Eucharist. But even he was very accustomed to the governing Benedictine tradition of the time. What was that?
The tabernacle was in a chapel where the monks prayed. The mass was celebrated in the abbey Church. They were literally two separate spaces. The practice was carried outside of the monasteries to the cathedral parishes. The monks had the custom of bowing before the altar at the beginning, during and at the end of the mass. That’s where bowing before the altar came from. They did not bow to the altar outside of mass times, probably because they were not in the abbey church, except for mass.
Where did the genuflection come from? The ancient custom was to kiss the floor when entering the presence of the Blessed Sacrament. Most lay people rarely saw this, because the Blessed Sacrament was not in the main church.
When the Franciscans took the monastic traditions outside of the enclosure things began to develop rather quickly. Like the monks before them, the friars kissed the floor when they entered the chapel, because the tabernacle was on the main altar. The chapel was the place where they prayed the Liturgy of the Hours and where they celebrated mass. They did not have two spaces as the monks had.
It was a very common custom of the time to kneel before the lord (king, duke, count, etc). While the friars went down and kissed the floor, the laity tried to follow suit with the reverence that they knew. They genuflected, as they had done for the lords of their day and ancient days before that.
As we see, the two customs of bowing to the altar and prostrating before the Blessed Sacrament become blended when the tabernacle and the altar of sacrifice are united in the same space.
The Franciscans and Dominicans grew, almost explosively. Soon both orders had friars and friaries all over Europe. Both professed very austere poverty. Both had very small places of worship, unlike the Benedictine and later the Cistercians who had large monastic complexes.
As the Franciscans and Dominicans replaced the Benedictines and the Cistercians pulled the Benedictine tradition back into the enclosure, the laity became more Franciscan and Dominican by default. Eventually, what had been practices that were particular to those two orders, Franciscans and Dominicans, became common practices in the Church. As new religious communities were founded, they did what the early Franciscans had done. The Franciscans took from the Benedictine abbey and the other communities took from the Franciscans and Dominicans. Eventually, all of those apostolic communities that were opening churches and parishes around Europe carried with them the custom of a tabernacle on the main altar. It ceased to be a mendicant custom and became common to a larger pool of people, thanks to the missionary work of all these active religious communities that followed.
Once you had the tabernacle fairly well established on the main altar, the custom of bowing to the altar at the beginning, middle and end of mass, was lost. It was replaced by what we still call the “venia” today. To us, friars, it is kissing the floor. To the laity it was going doing on the knee, at least.
Of course, in St. Mary’s Cathedral here in Halifax, the Tabernacle is just to the left of the altar (still in a prominent position, thank God). The layout of the cathedral means that one must go up the aisle to the altar, then move to the left (up the Sanctuary steps) to approach the Tabernacle, then left again into the western wall to get into the Sacristy. This requires one to bow before the altar and then genuflect before the Tabernacle, if one wishes to enter the Sacristy.