  Bound states in the continuum (BICs) in photonic crystal slabs represent the
resonances with an infinite quality(Q)-factor, occurring above the light line
for an infinitely periodic structure. We show that a set of BICs can turn into
quasi-BICs with a very high Q-factor even for two or three unit cell
structures. They are explained by a viewpoint of BICs originating from the
tight binding of individual resonances of each unit cell as in semiconductors.
Combined with a reciprocal-space matching technique, the microcavities based on
quasi-BICs can achieve a Q-factor as high as defect-based PhC microcavities.
These results may enable experimental studies of BICs in a compact platform as
well as realizing a new concept of high-Q mirrorless microcavities.
