Communion under both kinds

Communion under both kinds in Christianity is the reception under both "species" (i.e., both the consecrated bread and wine) of the Eucharist.

In reference to the Eucharist as a sacrifice, Communion under both kinds belongs at least to the integrity and essence, of the rite, and may not be omitted without violating the precept of Christ: "Do this for a commemoration of me" (Luke 22:19). This is mentioned implicitly by the Council of Trent (Sess. XXI, c. i; XXII, c. i), and the General Instruction of the Roman Missal states that the people "...should share the cup when it is permitted. Then, Communion is a clearer sign of sharing in the sacrifice that is actually being celebrated."

In modern Catholic practice in Europe and much of the English-speaking world, Ireland excluded, Communion is offered under both kinds when receiving at Mass. However, Catholicism teaches that Christ is sacramentally (and equally) present under each species, and therefore if a person receives only one species, Christ is fully present and nothing is lacking.

In the Early Church, Communion was ordinarily administered and received under both kinds. That such was the practice mentioned by Paul in I Corinthians 11:28. But side by side in the Early Church there existed the custom of communicating in certain cases under one kind alone e.g. when people took home some of the Eucharist after Sunday worship and communicated during the week and also when the Eucharist was brought to the sick.

By the Middle Ages, the Church had become, like most of European society, increasingly hierarchical. There was much stress on being holy when receiving Communion, and a greatly heightened appreciation of the sufferings of Christ. This meant that all who approached the altar were to be as pure as possible, and inevitably led to the exclusion of the laity from administering the Eucharist, reserving the practice to the clergy. It is difficult to say when the practice of offering the chalice to the people stopped, but it may be presumed that this was part of the way in which Church authorities sought to prevent anything disrespectful happening to the Eucharist; it was also, by this time, that Communion was given only on the tongue.

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Wikipedia